Support custom config when running gossip ref tests

[StefanBratanov]

For example for this reference test https://github.com/StefanBratanov/consensus-spec-tests/tree/master/tests/mainnet/fulu/networking/gossip_beacon_block/pyspec_tests/gossip_beacon_block__valid_at_blob_parameters_limit we should load a different config.yml to test different blob parameters