test(files): improve window title mocking to account for Nightly changes

Details:
- Nightly (after PR number 28387 in neovim/neovim) started to truncate
  window titles from the left (instead of from the right). This is a
  good change, but it didn't work well with screenshot testing of
  'mini.files'.
  Those tests mock window title by replacing variable starting part of
  the path with a known one ("MOCK_ROOT"). As mocking was not very
  advanced, it started to result on Nightly in screenshots with floating
  window titles that get truncated from the left **after** mocking.

  The solution is to make mocking a bit more advanced: replace exactly
  the number of characters from "MOCK_ROOT" that are in visible part of
  path start.

Author: echasnovski

tests/dir-files/mock-win-functions.lua

@@ -1,7 +1,14 @@
 local mock_title = function(x)
   if type(x) ~= 'string' then return x end
-  -- Make sure that full path title is same on any machine
-  return x:gsub('^.*/tests/dir%-files', 'MOCK_ROOT/tests/dir-files')
+  -- Make sure that full path title is the same on any machine
+  return x:gsub('^(.+)/tests/dir%-files', function(m)
+    -- Account for possible title truncation.
+    -- NOTE: This will also remove the intentional truncation with '…' prefix.
+    -- There should be dedicated tests for this truncation that are not
+    -- affected by this mock.
+    local mocked_root = string.sub('MOCK_ROOT', -vim.fn.strdisplaywidth(m))
+    return mocked_root .. '/tests/dir-files'
+  end)
 end
 
 _G.nvim_open_win_orig = vim.api.nvim_open_win

tests/screenshots/tests-test_files.lua---Preview---does-not-highlight-big-files

@@ -1,5 +1,5 @@
 --|---------|---------|---------|---------|---------|---------|---------|---------|
-01|┌MOCK_ROOT/tests/dir-files┐┌ big.lua ────────────────┐                          
+01|┌OCK_ROOT/tests/dir-files ┐┌ big.lua ────────────────┐                          
 02|│ common                 ││local a = "aaaaaaaaaaaaaa│                          
 03|│ lua                    │└─────────────────────────┘                          
 04|│ nested                 │                                                     

tests/screenshots/tests-test_files.lua---Preview---previews-only-one-level-deep

@@ -1,5 +1,5 @@
 --|---------|---------|---------|---------|---------|---------|---------|---------|
-01|┌MOCK_ROOT/tests/dir-files┐┌ dir-1 ──────────────────┐                          
+01|┌T/tests/dir-files/nested ┐┌ dir-1 ──────────────────┐                          
 02|│ dir-1                  ││ dir-11                 │                          
 03|└─────────────────────────┘│ dir-12                 │                          
 04|~                          └─────────────────────────┘                          

tests/screenshots/tests-test_files.lua---Preview---works-for-directories

@@ -1,5 +1,5 @@
 --|---------|---------|---------|---------|---------|---------|---------|---------|
-01|┌MOCK_ROOT/tests/dir-files┐┌ .a-dir ─────────────────┐                          
+01|┌T/tests/dir-files/common ┐┌ .a-dir ─────────────────┐                          
 02|│ .a-dir                 ││ aa-file                │                          
 03|│ a-dir                  ││ ab-file                │                          
 04|│ b-dir                  │└─────────────────────────┘                          

tests/screenshots/tests-test_files.lua---Preview---works-for-directories-002

@@ -1,5 +1,5 @@
 --|---------|---------|---------|---------|---------|---------|---------|---------|
-01|┌MOCK_ROOT/tests/dir-files┐┌ a-dir ──────────────────┐                          
+01|┌T/tests/dir-files/common ┐┌ a-dir ──────────────────┐                          
 02|│ .a-dir                 ││ aa-dir                 │                          
 03|│ a-dir                  ││ aa-file                │                          
 04|│ b-dir                  ││ ab-file                │                          

tests/screenshots/tests-test_files.lua---Preview---works-for-files

@@ -1,5 +1,5 @@
 --|---------|---------|---------|---------|---------|---------|---------|---------|
-01|┌MOCK_ROOT/tests/dir-files┐┌ a.lua ──────────────────┐                          
+01|┌OOT/tests/dir-files/real ┐┌ a.lua ──────────────────┐                          
 02|│ a.lua                  ││local a = 1              │                          
 03|│ b.txt                  ││local t = {              │                          
 04|│ c.gif                  ││  x = math.max(1, 2),    │                          

tests/screenshots/tests-test_files.lua---Preview---works-for-files-002

@@ -1,5 +1,5 @@
 --|---------|---------|---------|---------|---------|---------|---------|---------|
-01|┌MOCK_ROOT/tests/dir-files┐┌ b.txt ──────────────────┐                          
+01|┌OOT/tests/dir-files/real ┐┌ b.txt ──────────────────┐                          
 02|│ a.lua                  ││Line 1                   │                          
 03|│ b.txt                  ││Line 2                   │                          
 04|│ c.gif                  ││Line 3                   │                          

tests/screenshots/tests-test_files.lua---Preview---works-for-files-003

@@ -1,5 +1,5 @@
 --|---------|---------|---------|---------|---------|---------|---------|---------|
-01|┌MOCK_ROOT/tests/dir-files┐┌ c.gif ──────────────────┐                          
+01|┌OOT/tests/dir-files/real ┐┌ c.gif ──────────────────┐                          
 02|│ a.lua                  ││-Non-text-file-----------│                          
 03|│ b.txt                  │└─────────────────────────┘                          
 04|│ c.gif                  │                                                     

tests/screenshots/tests-test_files.lua---Preview---works-for-files-004

@@ -1,5 +1,5 @@
 --|---------|---------|---------|---------|---------|---------|---------|---------|
-01|┌MOCK_ROOT/tests/dir-files┐┌ LICENSE ────────────────┐                          
+01|┌OOT/tests/dir-files/real ┐┌ LICENSE ────────────────┐                          
 02|│ a.lua                  ││                         │                          
 03|│ b.txt                  │└─────────────────────────┘                          
 04|│ c.gif                  │                                                     

tests/screenshots/tests-test_files.lua---Preview---works-for-files-005

@@ -1,5 +1,5 @@
 --|---------|---------|---------|---------|---------|---------|---------|---------|
-01|┌MOCK_ROOT/tests/dir-files┐┌ Makefile ───────────────┐                          
+01|┌OOT/tests/dir-files/real ┐┌ Makefile ───────────────┐                          
 02|│ a.lua                  ││VAR ?= 1                 │                          
 03|│ b.txt                  ││                         │                          
 04|│ c.gif                  ││all: test                │