certtool: crl handling updates

This improves the messages printed by certtool in CRL generation cases and adds a sanity test for the validation of CRLs.

Checklist

• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• No obvious mistakes in the code

Relates #564 (closed)