test: remove unnecessary nil argument to testutil (#28270)

test/functional/ui/embed_spec.lua

@@ -233,7 +233,7 @@ describe('--embed UI', function()
     }
     eq({ [16777215] = true }, seen)
 
-    -- NB: by accident how functional/t.lua currently handles the default color scheme, the
+    -- NB: by accident how functional/testutil.lua currently handles the default color scheme, the
     -- above is sufficient to test the behavior. But in case that workaround is removed, we need
     -- a test with an explicit override like below, so do it to remain safe.
     startup('--cmd', 'hi NORMAL guibg=#FF00FF')

test/functional/ui/output_spec.lua

@@ -81,7 +81,7 @@ describe('shell command :!', function()
       {3:-- TERMINAL --}                                    |
     ]],
       {
-        -- test/functional/t.lua defaults to background=light.
+        -- test/functional/testutil.lua defaults to background=light.
         [1] = { reverse = true },
         [3] = { bold = true },
         [10] = { foreground = 2 },

test/functional/ui/screen.lua

@@ -70,7 +70,7 @@
 -- To help write screen tests, see Screen:snapshot_util().
 -- To debug screen tests, see Screen:redraw_debug().
 
-local t = require('test.functional.testutil')(nil)
+local t = require('test.functional.testutil')()
 local busted = require('busted')
 local deepcopy = vim.deepcopy
 local shallowcopy = t.shallowcopy