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.
also make implicit submodules "uri" and "_inspector" work with completion
this is needed for `:lua=vim.uri_<tab>` wildmenu completion
to work even before uri or _inspector functions are used.
