Skip tests that cannot be run

If a test requires Galera but it is not available, it is now skipped. All
tests that explicitly use the `test.galera` member as well as all tests
that use a Galera configuration now require that Galera is present.

maxscale-system-test/galera_priority.cpp

@@ -163,6 +163,7 @@ int simple_failover(TestConnections* test)
 
 int main(int argc, char** argv)
 {
+    TestConnections::require_galera(true);
     TestConnections* test = new TestConnections(argc, argv);
     test->galera->verbose = false;
     int rval1 = 0;