tests: convert oath tests to pytest

[noor-latif]

I got the impression unittest and pytest where used interchangeably, and I figured maybe pytest was more favorable for you guys. I made some suggested refactoring. Maybe the re-factor was a bit large.

Summary

• drop unittest.TestCase usage in tests/test_oath.py in favor of standalone pytest functions
• use parametrization for credential parsing, formatting, derivation, and issuer parsing cases to reduce repetition
• keep expected values identical

⚡Tests seems to succeed like before.