[FIXED JENKINS-37189] Only append `?uctest` to http/https update center URLs

- Also fixes to append `&uctest` if the update center URL already contains a query fragment

core/src/main/java/hudson/model/UpdateCenter.java

@@ -1042,7 +1042,11 @@ public class UpdateCenter extends AbstractModelObject implements Saveable, OnMas
          * @throws IOException if a connection to the update center server can't be established.
          */
         public void checkUpdateCenter(ConnectionCheckJob job, String updateCenterUrl) throws IOException {
-            testConnection(new URL(updateCenterUrl + "?uctest"));
+            if (updateCenterUrl.startsWith("http://") || updateCenterUrl.startsWith("https://")) {
+                testConnection(new URL(updateCenterUrl + (updateCenterUrl.indexOf('?') == -1 ? "?uctest" : "&uctest")));
+            } else {
+                testConnection(new URL(updateCenterUrl));
+            }
         }
 
         /**