Fix watchdog test

The test needs to remove the expected core dump to prevent the test from
failing.

maxscale-system-test/mxs2057_systemd_watchdog.cpp

@@ -65,6 +65,7 @@ void test_watchdog(TestConnections& test, int argc, char* argv[])
         if (test.global_result == 0)
         {
             test.tprintf("Maxscale was killed by systemd - ok");
+            test.maxscales->ssh_node_f(0, true, "rm -f /tmp/*core");
         }
     }
 }