Auth tests: do not hardcode port 9999
Fix code added in f5c1efc5 (Add tls configuration to gitaly golang server, 2018-12-04) to stop hardcoding the net.Listen() port. Every other test that does this uses the API's feature to pick a free port.
Fix code added in f5c1efc5 (Add tls configuration to gitaly golang server, 2018-12-04) to stop hardcoding the net.Listen() port. Every other test that does this uses the API's feature to pick a free port.