TEST_PING doesn't remove the discovery info for a node if findInitialMembers() is called after stop()

Because of https://issues.jboss.org/browse/JGRP-1336, sometimes a node is "resurrected" and TEST_PING.findInitialMembers() is called even after the channel has been stopped.

If that happens the node is not removed from the discovery registry and the next node to start from the same test will hang trying to contact the dead node and elect a coordinator.