WIP: Add some editor tests (#3285)

This is a continuation of
https://github.com/zed-industries/zed/pull/3232, however as I started
working on this it seemed a bit premature.

There are a few GPUI changes to finish getting the editor green (mainly
clipboard and simulate_resize), as well as several editor methods to
port (several around changing selections), as well as several editor
methods that are now fully broken (tagged with 'todo(finish editor
tests)').

I am also comitting the partially ported integration tests, which where
close to compiling.

Release Notes:

- N/A