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/mxs1751_available_when_donor_crash.cpp

@@ -15,6 +15,7 @@
 
 int main(int argc, char* argv[])
 {
+    TestConnections::require_galera(true);
     TestConnections test(argc, argv);
 
     for (int i = 0; i < 2; i++)