Wait for multi-user.target in remote maintenance test

It seems like the test may otherwise run too early, not everything being
yet set up.

testing/integration/remote-maintenance.nix

@@ -51,6 +51,7 @@ pkgs.nixosTest {
 
     # Auto-connected makes ZT operational on its own
     autoConnected.start()
+    defaultConnectionSetting.wait_for_unit('multi-user.target')
     is_zt_operational(autoConnected)
   '';
 }