Improve test speed

[JoeZiminski]

PR #635 investigated an issue in which tests suddenly became very slow (fixed in #641). However, it also highlighted some changes that can generally speed up the tests and help troubleshooting:

1. add durations argument to pytest in the workflows to print how long each test takes
2. focus on the TUI tests, which are by far the slowest
3. removing codecov actually decreases test speed by like 5 minutes, but it is almost certainly not worth removing this.

[adamltyson]

Any idea why codcov makes a difference? All it should do is upload an xml (I think) file to a server somewhere.