validation: Don't use IsValid() to filter for invalid blocks

IsValid() also returns false for blocks that have not been
validated yet up to the default validity level of BLOCK_VALID_TRANSACTIONS but
are not marked as invalid - e.g. if we only know the header.
Here, we specifically want to filter for invalid blocks.

Also removes the default arg from IsValid() which is now unused outside
of tests, to prevent this kind of misuse for the future.

Co-authored-by: TheCharlatan <seb.kung@gmail.com>

src/test/fuzz/chain.cpp

@@ -30,7 +30,7 @@ FUZZ_TARGET(chain)
         (void)disk_block_index->GetMedianTimePast();
         (void)disk_block_index->GetUndoPos();
         (void)disk_block_index->HaveNumChainTxs();
-        (void)disk_block_index->IsValid();
+        (void)disk_block_index->IsValid(BLOCK_VALID_TRANSACTIONS);
     }
 
     const CBlockHeader block_header = disk_block_index->GetBlockHeader();