Patch to maybe show testing outputs if not in reference based on verbosity.

[emin63]

This is a simple patch to make it so that if you run pytest with -vvv and you have unexpected output in stderr when testing the notebook, you will see the contents of stderr.

Without this patch, you just get a test failure telling you:

     Unexpected output fields from running code: {'stderr'}

but without showing you what is in stderr. This can be annoying if you are running an a CI environment like GitHub which has a different environment than your local system and is harder to debug minor warnings or other errors.

[takluyver]

I think something like this makes sense, though I wonder if we want to show it at a lower verbosity level than -vvv, or even by default. It might also be necessary to do something about very long outputs.