Extend test cases for selectors with multivalued selections

[jhrozek]

Summary

I realized we don't have any test case for multiple selectors, let's fix
that

Fixes: #4016

Change Type

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

make test

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 54.51% (+0.01%) from 54.497%
when pulling 4a0a64d on jhrozek:selectors_test_multi
into 95535e5 on stacklok:main.