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/core/exit_spec.lua

@@ -1,24 +1,25 @@
-local t = require('test.functional.testutil')()
+local t = require('test.testutil')
+local n = require('test.functional.testnvim')()
 
-local assert_alive = t.assert_alive
-local command = t.command
-local feed_command = t.feed_command
-local feed = t.feed
-local eval = t.eval
+local assert_alive = n.assert_alive
+local command = n.command
+local feed_command = n.feed_command
+local feed = n.feed
+local eval = n.eval
 local eq = t.eq
-local run = t.run
-local fn = t.fn
-local nvim_prog = t.nvim_prog
+local run = n.run
+local fn = n.fn
+local nvim_prog = n.nvim_prog
 local pcall_err = t.pcall_err
-local exec_capture = t.exec_capture
-local poke_eventloop = t.poke_eventloop
+local exec_capture = n.exec_capture
+local poke_eventloop = n.poke_eventloop
 
 describe('v:exiting', function()
   local cid
 
   before_each(function()
-    t.clear()
-    cid = t.api.nvim_get_chan_info(0).id
+    n.clear()
+    cid = n.api.nvim_get_chan_info(0).id
   end)
 
   it('defaults to v:null', function()
@@ -74,7 +75,7 @@ describe(':cquit', function()
   end
 
   before_each(function()
-    t.clear()
+    n.clear()
   end)
 
   it('exits with non-zero after :cquit', function()