doctest: Allow checking for Traceback context errors

Feature or enhancement

Proposal:

If the error thrown in a doctest command is a ValueError, and that error object has a __context__ of RuntimeError, it would be nice to be able to do this:

>>> function_that_raises()
Traceback (most recent call last):
  ...
RuntimeError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  ...
ValueError

And a similar thing for __cause__ but with the different line of text in the middle.

Has this already been discussed elsewhere?

I have already discussed this feature proposal on Discourse

Links to previous discussion of this feature:

I did make a discourse post, but it only got a like and no comments :)

Start with the doctest traceback-checking implementation and its existing tests; the issue provides no file paths. Reproduce the shown context case and compare it with the proposed cause variant, then add coverage for the requested intermediate traceback text and verify the doctest output.