test: improve test conventions

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

test/functional/lua/text_spec.lua

@@ -1,6 +1,6 @@
-local helpers = require('test.functional.helpers')(after_each)
-local clear = helpers.clear
-local eq = helpers.eq
+local t = require('test.functional.testutil')(after_each)
+local clear = t.clear
+local eq = t.eq
 
 describe('vim.text', function()
   before_each(clear)