test: add gossip_proposer_attestation=True to fix test_reorg_on_newly_justified_slot

[unnawut]

🗒️ Description

A test vector (test_reorg_on_newly_justified_slot) is still failing even after the key regen in #455. This is caused by a spec's behavior change in #449 where we no longer embed the proposer's attestation as part of the block proposal (they're now gossiped like non-proposer's attestations) so a block proposal no longer carries 1 attestation weight from the proposer so fork A never gets any fork choice weight (so it was using lexicographical order hence the random pass/failure).

This PR is a quick fix to get the test vectors working again. A better solution might be to make gossip_proposer_attestation=True the default behavior for all test cases but that's a much bigger change.

✅ 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.

[Kayden-ML]

Looks good 👍