[Upgrade] Use pytest-datadir to manage data files for tests

[wfondrie]

@jspaezp suggested switching over to pytest-datadir for managing our test data. This would have the benefit of simplifying our test fixtures, making it easier for folks to contribute. The user impact of this change is minimal, but it does add an additional developer dependency.

I think this change is probably worth making.

[jspaezp]

I think i could tackle this issue but I feel like the cross linking PSMs PR should be solved first ... but if you want to set that aside and re-base it with the new test stuff i could go ahead and start the changes.

Best!
Sebastian

[wfondrie]

Unfortunately, cross-linking support has been kicked down the road a bit, so you shouldn't worry about. If you want to tackle the test conversion to pytest-datadir, please feel free to go ahead!