ci/cd: adding tests for custom trust config

[SequeI]

Summary

Adding tests for the new custom sigstore instance addition, just to ensure everything stays working as intended, and is continously tested with each new codebase addition

Checklist

• All commits are signed-off, using DCO
• All new code has docstrings and type annotations
• All new code is covered by tests. Aim for at least 90% coverage. CI is configured to highlight lines not covered by tests.
• Public facing changes are paired with documentation changes
• Release note has been added to CHANGELOG.md if needed

[SequeI]

Odd Hatch install failures, otherwise seems fine.

[mihaimaruseac]

Will look at the tests later, but wanted to submit the code changes first

[SequeI]

Tests failing for a bunch of different reasons, not due to PR :)

EDIT: failures fixed in #575