nixos/tests/jenkins: check if jenkins is available

nixos/tests/jenkins.nix

···
     startAll;
 
     $master->waitForUnit("jenkins");
+
+    $master->mustSucceed("curl http://localhost:8080 | grep 'Authentication required'");
+
     print $master->execute("sudo -u jenkins groups");
     $master->mustSucceed("sudo -u jenkins groups | grep jenkins | grep users");
 
···
 
     $slave->mustFail("systemctl is-enabled jenkins.service");
   '';
-})
+})