test: set 'termguicolors' in outer Nvim instance (#26437)

Currently, the value of $COLORTERM in :terminal in tests depends on
outer environment because of 'notermguicolors'.

If $COLORTERM is not set in :terminal, an inner Nvim instance will try
to detect 'termguicolors' support, which may interfere with tests.

So set 'termguicolors' in outer Nvim instance unless $COLORTERM needs to
be overridden, and unset it in inner Nvim instance when running TUI.

1 files changed, 6 insertions, 5 deletions

diff --git a/test/functional/helpers.lua b/test/functional/helpers.lua
index 5780d0a978..7ec67a2cb3 100644
--- a/test/functional/helpers.lua
+++ b/test/functional/helpers.lua
@@ -28,17 +28,18 @@ module.nvim_prog = (
 )
 -- Default settings for the test session.
 module.nvim_set = (
-  'set shortmess+=IS background=light noswapfile noautoindent startofline'
+  'set shortmess+=IS background=light termguicolors noswapfile noautoindent startofline'
   ..' laststatus=1 undodir=. directory=. viewdir=. backupdir=.'
-  ..' belloff= wildoptions-=pum joinspaces noshowcmd noruler nomore redrawdebug=invalid'
-  ..' notermguicolors')
+  ..' belloff= wildoptions-=pum joinspaces noshowcmd noruler nomore redrawdebug=invalid')
 module.nvim_argv = {
   module.nvim_prog, '-u', 'NONE', '-i', 'NONE',
   -- XXX: find treesitter parsers.
   '--cmd', runtime_set,
   '--cmd', module.nvim_set,
-  '--cmd', 'mapclear',
-  '--cmd', 'mapclear!',
+  -- Remove default mappings.
+  '--cmd', 'mapclear | mapclear!',
+  -- Unset $COLORTERM so that it won't propagate to :terminal when 'notermguicolors'.
+  '--cmd', 'unlet $COLORTERM',
   -- Make screentest work after changing to the new default color scheme
   -- Source 'vim' color scheme without side effects
   -- TODO: rewrite tests