vim test redux (#11709)

This cleans up the neovim-backed vim tests:
- removed exempted tests (we'll rely on bug reports to find missing edge
cases)
- moved all assertions into non-async fn's so that failures are
reporting on the right file/line
- removed the NeovimBackedBindingTestContext
- renamed a few things to make them clearer
- reduced the number of permutations tested in some cases to reduce
slowest test from 60s to 5s

Release Notes:

- N/A