test: convert some unittest based tests to pytest

[aucampia]

Summary of changes

Fairly straight forward conversion, converted one skip to an xfail
though with an explanation of what is going wrong.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

Coverage remained the same at 90.447% when pulling 20181c5 on aucampia:iwana-20220805T2353-more_pytest into cc80c9c on RDFLib:master.

[ghost]

👍