fix(tests): only get the color map once, even for multiple test files

Problem: test/functional/ui/screen.lua would be reloaded for each
*_spec.lua file, which causes an extra nvim session to be started
to get the color map each time.

solution: Mark screen.lua as a preloaded file, but defer the
loading of the color map to the first time Screen object is initialised.

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 helpers = require('test.functional.helpers')(nil)
+local screen = require('test.functional.ui.screen')
 local busted = require("busted")
 local is_os = helpers.is_os