tests: add test vectors for aggregated signatures

[unnawut]

🗒️ Description

Adds 2 tests for verifying aggregated signatures:

• test_proposer_and_attester_signatures (existing but updated to use aggregated signatures)
• test_invalid_aggregated_attestation_signature (added)

I have at least 1 more test vector to add but will submit as a separate PR once this one is merged, because the diff here is already becoming hard to read:

• a negative test that rejects aggregated attestations with valid signatures, but one of the valid signatures doesn't correspond to the correct validator

🔗 Related Issues or PRs

Follow #318

✅ Checklist

• Ran tox checks to avoid unnecessary CI fails:

  uvx tox

• Considered adding appropriate tests for the changes.
• Considered updating the online docs in the ./docs/ directory.

[unnawut]

@anshalshukla @shariqnaiyer Couldn’t add you as a reviewer somehow but I’m asking for permission to be fixed