Add test vectors for store.on_gossip_attestation()

[unnawut]

We don't yet have any test vectors for fork choice store's on_gossip_attestation() which is important for attestation validation and storage.

The test vectors should at least cover the validation rules in validate_attestation() that eventually gets called by on_gossip_attestation()

[Smartdevs17]

I would like to work on this issue. Planning to add test vectors for store.on_gossip_attestation() covering the validation rules in validate_attestation().

[tcoratger]

I would like to work on this issue. Planning to add test vectors for store.on_gossip_attestation() covering the validation rules in validate_attestation().

Sure, assigned.