Add gossip validation spec tests for proposer/attester slashings (#9323)

Addresses #9232 partially. This PR covers two topics only.
* #9232

Wires up networking test vectors for `gossip_proposer_slashing` and `gossip_attester_slashing` topics.

The tests also revealed minor spec non-compliance where invalid slashings were ignored rather than rejected.


  - Refactor `process_gossip_proposer_slashing` and `process_gossip_attester_slashing` to return `MessageAcceptance`, so it can be verified in the tests
- Add `GossipValidation` test case, handler, and test entries
- Spec compliance fix: distinguish between internal errors and validation error - return `Reject` when the slashing is invalid and only penalise on invalid messages


Co-Authored-By: Jimmy Chen <jchen.tc@gmail.com>

testing/ef_tests/check_all_files_accessed.py

@@ -75,8 +75,15 @@ excluded_paths = [
     "tests/.*/compute_challenge/.*",
     # We don't need these manifest files at the moment.
     "tests/.*/manifest.yaml",
-    # TODO: gossip condition tests not implemented yet
-    "tests/.*/.*/networking/.*",
+    # TODO: Remaining gossip validation topics not yet implemented
+    "tests/.*/.*/networking/gossip_beacon_block/.*",
+    "tests/.*/.*/networking/gossip_beacon_attestation/.*",
+    "tests/.*/.*/networking/gossip_beacon_aggregate_and_proof/.*",
+    "tests/.*/.*/networking/gossip_voluntary_exit/.*",
+    "tests/.*/.*/networking/gossip_bls_to_execution_change/.*",
+    "tests/.*/.*/networking/gossip_sync_committee_message/.*",
+    "tests/.*/.*/networking/gossip_sync_committee_contribution_and_proof/.*",
+    "tests/.*/.*/networking/gossip_blob_sidecar/.*",
     # TODO: fast confirmation rule not merged yet
     "tests/.*/.*/fast_confirmation",
 ]