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/lua/iter_spec.lua

@@ -1,4 +1,5 @@
-local t = require('test.functional.testutil')()
+local t = require('test.testutil')
+
 local eq = t.eq
 local matches = t.matches
 local pcall_err = t.pcall_err