test: set 'termguicolors' for every child process

Details:
- This seems to make testing more robust. Not in the last because
  'mini.hues' defines only `gui{fg,bg,sp}` highlight attributes.

- The driver for this change was PR number 35026 on neovim/neovim.
  It adjusted how highlight groups linked to `Normal` are not displayed
  over `CursorLine`. This is a good change, but introduced an issue with
  `child.fn.screenattr()` output.
  It seems to started to ignore not only `linked to Normal` groups,
  but at least some others (like `linked to NormalFloat`, which has
  different background than `Normal`).

  The visible side effect here was that current line in 'mini.files'
  explorer (which has cursorline enabled) was shown with a single
  highlight from start to finish where two (from `content.highlight` and
  `CursorLine`) was expected. Two expected groups have visibly different
  background when tried to reproduce interactively.

  What seems to be the culprit is that Neovim's C code in this case now
  acts differently based on the output of `ui_rgb_attached()`.
  Child processes do not have an attached UI that explicitly says that
  it supports `rgb`. Nor there is an explicit 'termguicolors'.

  Explicitly enabling 'termguicolors' fixes the 'mini.files' test.

Author: echasnovski

scripts/minimal_init.lua

@@ -17,3 +17,12 @@ end
 
 -- Ensure no custom fold method in Lua files (it interfers with many tests)
 vim.cmd('au FileType lua set foldmethod=manual')
+
+-- - Ensure that child process is tested with termguicolors, since 'mini.hues'
+--   only works with it. This might matter with screenshot testing.
+--   One example is how https://github.com/neovim/neovim/pull/35026 adjusts
+--   the behavior based on the presence of `rgb`.
+--   NOTE: similar effect can be achieved by adding the following to
+--   `child.setup` in 'helpers.lua':
+--   `child.api.nvim_ui_attach(child.o.columns, child.o.lines, { rgb=true })`
+vim.o.termguicolors = true

tests/test_basics.lua

@@ -148,8 +148,6 @@ T['Options'] = new_set()
 T['Options']['work'] = function()
   -- Basic options (should be set by default)
   eq(child.g.mapleader, vim.NIL)
-  -- - `termguicolors` is enabled in Neovim=0.10 by default (if possible)
-  if child.fn.has('nvim-0.10') == 0 then eq(child.o.termguicolors, false) end
   eq(child.o.number, false)
   eq(child.o.signcolumn, 'auto')
   eq(child.o.fillchars, '')
@@ -160,7 +158,6 @@ T['Options']['work'] = function()
   load_module()
 
   eq(child.g.mapleader, ' ')
-  if child.fn.has('nvim-0.10') == 0 then eq(child.o.termguicolors, true) end
   eq(child.o.number, true)
   eq(child.o.signcolumn, 'yes')
   eq(child.o.fillchars, 'eob: ')