WIP: [POC] Refactor: port unittest to pytest

[Mikaayenson]

Issues

Related to https://github.com/elastic/ia-trade-team/issues/238

Sets up some foundational changes that we plan to make to support Detections as Code. We want to be able to use more useful features of pytest and the current setup of intermingling python unittest while we actually already use pytest complicated our ability to do this. Migrating to a single testing capability removes some of this confusion and sets us up for the next enhancements to come.

Summary

• Refactors the test suite from unittest to pytest. This is this first of several PRs to come. Initially I wanted to add in the pytest mark labels, but we'll save that for a subsequent PR to limit the scope of changes.
• Migrates base.py to conftests.py (pytest magic file)
• Also found a bug in one of the unittest TestPackages::test_rule_versioning where we weren't actually iterating over the rules.

Testing

Unit tests should all still pass as before.

[terrancedejesus]

Port seems fine. Checked out branch, ran tests collectively and then individually and they worked.

[eric-forte-elastic]

Ports look good! Tests work locally.

[Mikaayenson]

FWIW, I manually compared the time it took to run the unit tests with other PRs and the time to complete has not changed.

[Mikaayenson]

Converted to draft so it doesnt accidentally get merged.