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/vimscript/map_functions_spec.lua

@@ -1,17 +1,18 @@
-local t = require('test.functional.testutil')()
+local t = require('test.testutil')
+local n = require('test.functional.testnvim')()
 
-local clear = t.clear
+local clear = n.clear
 local eq = t.eq
-local eval = t.eval
-local exec = t.exec
-local exec_lua = t.exec_lua
-local expect = t.expect
-local feed = t.feed
-local fn = t.fn
-local api = t.api
-local source = t.source
-local command = t.command
-local exec_capture = t.exec_capture
+local eval = n.eval
+local exec = n.exec
+local exec_lua = n.exec_lua
+local expect = n.expect
+local feed = n.feed
+local fn = n.fn
+local api = n.api
+local source = n.source
+local command = n.command
+local exec_capture = n.exec_capture
 local pcall_err = t.pcall_err
 
 describe('maparg()', function()