Fix failures in real_encode_with_server.

Fix failures in real_encode_with_server.

This test suddenly started failing because the server is never found,
so never encodes any frames.

It's a mystery why this started failing, and even more of a mystery
why there seem to be two distinct reasons.

Some failures were apparently due to firewalld blocking UDP broadcast;
disabling the firewall would make the test pass again.

Other failures were apparently caused by an old EncodeServerFinder
hanging around, presumably finding servers before a new one had the
chance.