Add a timeout to mxs1585

The test will hang if one of the threads doesn't exit. To detect failures
faster, a timeout is needed.

maxscale-system-test/mxs1585.cpp

@@ -63,6 +63,7 @@ int main(int argc, char** argv)
     }
 
     running = false;
+    test.set_timeout(120);
 
     for (auto& a: threads)
     {