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/lots_of_rows.cpp

@@ -11,6 +11,7 @@
 
 int main(int argc, char* argv[])
 {
+    TestConnections::require_galera(true);
     TestConnections* Test = new TestConnections(argc, argv);
     char sql[10240];