conformance: Enable message set tests
conformance: Enable message set tests

Stop skipping the two tests around message sets as we have now patched
out the legacy feature `option message_set_wire_format = true;`, so the
tests will now pass.

Update the comments on the remaining tests as they now fail for other
reasons, not due to protolegacy being disabled.

alecthomas/protobuf,alecthomas/protobuf