Add basic testing
Add basic testing

- `test_extract_subjects`: given fake output from git status calls,
  render a status dashboard, then query for selected 'subjects' while
  moving the cursor.

  This is a complicated test, which ensures we render data as expected,
  and that our syntax has the scopes we expect not visually but as the
  interface we program against.

- `test_stage_files`: is an example of testing one of the many commands
  we actually have. Note that we select the whole buffer and ensure
  that only the stageable files and not any stashes are picked.

divmain/GitSavvy,divmain/GitSavvy,divmain/GitSavvy