Verify attestation justified_block_hash.

Previously there was not a check that the hash was in the chain, just
that it was known (in the database in any chain)

beacon_chain/validation/tests/block_validation/mod.rs

@@ -9,4 +9,4 @@ use super::ssz_helpers;
 use super::types;
 use super::validation;
 
-use super::attestation_validation::helpers::generate_attestation;
+use super::attestation_validation;