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 in neovim/neovim.
  It adjusted how highlight groups linked to `Normal` are not displayed
  over `CursorLine`. Although it had an issue (see issue number 36915 in
  neovim/neovim), this is a good change. The introduced issue was with
  highlighting in the floating window that had cursorline enabled.

  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: ')