test: simplify TUI bg-detection test

Previous approach skipped the test if the expected value matched the
default value ("dark"). New approach always checks, but uses retry() to
ignore potentially wrong 'background' before the terminal response is
handled.

test/functional/terminal/tui_spec.lua

@@ -859,14 +859,13 @@ describe('TUI background color', function()
 
   local function assert_bg(color, bg)
     it('handles '..color..' as '..bg, function()
-      feed_data('\027:autocmd OptionSet background :echo &background\n')
+      feed_data('\027]11;rgb:'..color..'\007')
-
+      -- Retry until the terminal response is handled.
-      -- Wait for the child Nvim to register the OptionSet handler.
+      retry(100, nil, function()
-      feed_data('\027:autocmd OptionSet\n')
+        feed_data(':echo &background\n')
-      screen:expect({any='--- Autocommands ---'})
+        screen:expect({
-
+          timeout=40,
-      feed_data('\012')  -- CTRL-L: clear the screen
+          grid=string.format([[
-      local expected_grid = [[
             {1: }                                                 |
             {4:~                                                 }|
             {4:~                                                 }|
@@ -874,17 +873,9 @@ describe('TUI background color', function()
             {5:[No Name]                       0,0-1          All}|
             %-5s                                             |
             {3:-- TERMINAL --}                                    |
-      ]]
+          ]], bg)
-      screen:expect(string.format(expected_grid, ''))
+        })
-
+      end)
-      feed_data('\027]11;rgb:'..color..'\007')
-      -- Because bg=dark is the default, we do NOT expect OptionSet event.
-      if bg == 'dark' then
-        screen:expect{unchanged=true,
-                      grid=string.format(expected_grid, '')}
-      else
-        screen:expect(string.format(expected_grid, bg))
-      end
     end)
   end