Add test case for ignore_external_masters + failover

The test checks that failover works even when the master of the monitored
cluster is a slave to an external masters. The test also verifies that the
servers do not get unexpected status labels.

maxscale-system-test/testconnections.h

@@ -531,4 +531,14 @@ void * timeout_thread(void *ptr );
  */
 void * log_copy_thread(void *ptr );
 
+/**
+* Dump two server status sets as strings
+*
+* @param current  The current status
+* @param expected The expected status
+*
+* @return String form comparison of status sets
+*/
+std::string dump_status(const StringSet& current, const StringSet& expected);
+
 #endif // TESTCONNECTIONS_H