test: add variants for blank prefixes and base

[aucampia]

Summary of changes

Test that the base directive and blank prefixes are handled correctly.

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: 90.851%. remained the same when pulling d61bf11 on aucampia:aucampia/20230602T2146-base_variants into 15a7c54 on RDFLib:main.