test: make preload of functional/testnvim.lua explicit (#35919)

Before functional/testnvim.lua was moved from functional/testutil.lua in
052498ed42, it was explicitly preloaded,
but now it is preloaded implicitly via functional/ui/screen.lua.

Also fix warnings about unused variables in other preload.lua files.

test/functional/preload.lua

@@ -2,6 +2,7 @@
 -- Busted started doing this to help provide more isolation.  See issue #62
 -- for more information about this.
 local t = require('test.testutil')
+require('test.functional.testnvim')()
 require('test.functional.ui.screen')
 
 local has_ffi, ffi = pcall(require, 'ffi')