TEST_PING should be protected against paralel cache manager startups

Caused by: java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextEntry(HashMap.java:810)
	at java.util.HashMap$ValueIterator.next(HashMap.java:839)
	at org.infinispan.test.fwk.TEST_PING.findMembers(TEST_PING.java:74)
	at org.jgroups.protocols.Discovery.findInitialMembers(Discovery.java:185)
	at org.jgroups.protocols.Discovery.down(Discovery.java:467)
	at org.jgroups.protocols.pbcast.NAKACK.down(NAKACK.java:593)
	at org.jgroups.protocols.UNICAST.down(UNICAST.java:432)
	at org.jgroups.protocols.pbcast.STABLE.down(STABLE.java:329)
	at org.jgroups.protocols.pbcast.ClientGmsImpl.findInitialMembers(ClientGmsImpl.java:207)
	at org.jgroups.protocols.pbcast.ClientGmsImpl.joinInternal(ClientGmsImpl.java:73)
	at org.jgroups.protocols.pbcast.ClientGmsImpl.join(ClientGmsImpl.java:38)
	at org.jgroups.protocols.pbcast.GMS.down(GMS.java:912)
	at org.jgroups.protocols.FC.down(FC.java:435)
	at org.jgroups.protocols.FRAG2.down(FRAG2.java:147)
	at org.jgroups.stack.ProtocolStack.down(ProtocolStack.java:1033)
	at org.jgroups.JChannel.down(JChannel.java:734)
	at org.jgroups.JChannel.connect(JChannel.java:296)
	... 21 more