test: improve test conventions

Work on https://github.com/neovim/neovim/issues/27004.

test/functional/preload.lua

@@ -1,10 +1,10 @@
 -- Modules loaded here will NOT be cleared and reloaded by Busted.
 -- Busted started doing this to help provide more isolation.  See issue #62
 -- for more information about this.
-local helpers = require('test.functional.helpers')(nil)
+local t = require('test.functional.testutil')(nil)
 require('test.functional.ui.screen')
 local busted = require('busted')
-local is_os = helpers.is_os
+local is_os = t.is_os
 
 if is_os('win') then
   local ffi = require('ffi')