[Feature Request] Better failure messages from `_wait_for` test methods

[alexcjohnson]

Currently hitting a CI-only test failure from wait_for_text_to_equal, works fine locally:

TimeoutException: Message: text -> a not found within 10s

I'd like to know what was found at the end of the timeout period instead of "a" but it's unclear to me how to massage the existing code to find that out.