refactor(tests): move lua-client into core and use it for functionaltests

Eliminates lua-client and non-static libluv as test time dependencies

Note: the API for a public lua-client is not yet finished.
The interface needs to be adjusted to work in the embedded loop
of a nvim instance (to use it to talk between instances)

test/helpers.lua

@@ -1,5 +1,5 @@
 require('test.compat')
-local shared = require('vim.shared')
+local shared = vim
 local assert = require('luassert')
 local busted = require('busted')
 local luv = require('luv')