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/mxs1980_blr_galera_server_ids.cpp

@@ -410,6 +410,7 @@ int main(int argc, char* argv[])
 {
     mxb::Log log(MXB_LOG_TARGET_STDOUT);
 
+    TestConnections::require_galera(true);
     TestConnections::skip_maxscale_start(true);
     TestConnections test(argc, argv);