Fix WebClient missing polyfill

Thanks to @basil for raising awareness of this - jenkinsci/jenkins#9148 (comment).

We're now using the JavaScript fetch() API for the builds widget which requires 
a polyfill for HtmlUnit. This polyfill is enabled by default as part of the Jenkins 
test harness createWebClient(), however in this case that method wasn't used 
as a couple of methods of WebClient needed to be overridden.

This MR enables the polyfill for that web client.

src/test/java/org/jenkinsci/plugins/authorizeproject/testutil/AuthorizeProjectJenkinsRule.java

@@ -56,7 +56,7 @@ public AuthorizeProjectJenkinsRule(
 
     @Override
     public WebClient createWebClient() {
-        return new WebClient() {
+        WebClient webClient = new WebClient() {
             private static final long serialVersionUID = 3389654318647204218L;
 
             @Override
@@ -68,6 +68,8 @@ public void throwFailingHttpStatusCodeExceptionIfNecessary(WebResponse webRespon
                 super.throwFailingHttpStatusCodeExceptionIfNecessary(webResponse);
             }
         };
+        webClient.getOptions().setFetchPolyfillEnabled(true);
+        return webClient;
     }
 
     public void before() throws Throwable {