add fixture to kill all terminals after each test

[Zsailer]

The tests were failing in test_terminals.py on CI because terminals opened in one test persisted across other tests. Here, I've added a fixture to kill all terminals after each test.

This is a work in progress—it doesn't fix the issue yet, but it's a step in the right direction. I can verify on my branch that terminals are staying open between tests.

I'll remove the "Draft" label once I've fixed the issue.

[Zsailer]

Finally fixed our broken test!

This isn't the prettiest fix, but it does the job. It only applies to terminal tests, so I don't want to spend to much time trying to make a more elegant solution.

[Zsailer]

@blink1073, @kevin-bates, or @echarles — Do you mind doing a review of this? I'd love to get our CI working again.

[blink1073]

LGTM, thanks!

[Zsailer]

Thanks, @blink1073! Merging.

[kevin-bates]

Thanks Zach. I had looked earlier then got called away. Sounds like we really don't know my mac/3.7 wasn't stopping terminals in this case, but I'm fine with this approach. (We'll want to port the terminal culling PR from Notebook. 😉 😉)

[Zsailer]

I think they were stopping—it was just taking too long. So the terminals weren't completely closed between tests (race condition issues). This just adds an explicit await call to make sure all the terminals close before each test wraps up.

[echarles]

Thanks a lot @Zsailer for all what you do here.