test: improve test conventions

Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.

Closes https://github.com/neovim/neovim/issues/27004.

test/functional/script/text_utils_spec.lua

@@ -1,5 +1,7 @@
-local t = require('test.functional.testutil')()
-local exec_lua = t.exec_lua
+local t = require('test.testutil')
+local n = require('test.functional.testnvim')()
+
+local exec_lua = n.exec_lua
 local eq = t.eq
 
 local function md_to_vimdoc(text, start_indent, indent, text_width)
@@ -28,7 +30,7 @@ end
 
 describe('md_to_vimdoc', function()
   before_each(function()
-    t.clear()
+    n.clear()
   end)
 
   test('can render para after fenced code', {