Too slow a system isn't a test failure

If testing cannot be reliably performed, the test should be skipped to
minimize unnecessary work that non-deterministic failures cause.

maxscale-system-test/mxs173_throttle_filter.cpp

@@ -132,7 +132,7 @@ void gauge_raw_speed(TestConnections& test)
         std::ostringstream os;
         os << "The raw speed is too slow, " << rs.qps
            << "qps, compared to max_qps = " << max_qps << "qps for accurate testing.";
-        test.add_result(1, "%s", os.str().c_str());
+        exit(0);
     }
 }