Also, some fun test helpers

Co-Authored-By: Mikayla <mikayla@zed.dev>

You can now use .debug_selector() to make it possible for tests to find
a given element,
and .debug_bounds() to find the coordinates of where it was painted.


Release Notes:

- (Added|Fixed|Improved) ...
([#<public_issue_number_if_exists>](https://github.com/zed-industries/community/issues/<public_issue_number_if_exists>)).