Fixing intermittent timing failures for SendMessagesTestCase

https://issues.jboss.org/browse/WFLY-4060

I've been struggling with intermittent failures because of timing issue in SendMessageTestCase. There is more information about reasons of this in bz: https://bugzilla.redhat.com/show_bug.cgi?id=1160761

I'm "backporting" the fix to WildFly by this PR.