Issue 6769 - Avoid hardcoding the URL

pages/exceptions/xmlhttprequest.tmpl

 title = $xmlhttprequest Exception
 template = testcase
 
 {% set testcase_moreinfo = [
   ("Filter Options", "https://adblockplus.org/filters#options"),
 ] %}
 
 <script>
 function aborted(evt) {
   pagelog("XMLHTTPRequest was aborted.");
 }
 
 function failed(evt) {
   pagelog("XMLHTTPRequest failed.");
 }
 
 function makerequest() {
   var req = new XMLHttpRequest();
   req.addEventListener("error", failed);
   req.addEventListener("abort", aborted);
-  req.open("GET", "https://testpages.adblockplus.org/testcasefiles/xmlhttprequestexception/text.txt", true);
+  req.open("GET", "{{ site_url }}/testcasefiles/xmlhttprequestexception/text.txt", true);
   req.onload = function(e) {
     if (this.status == 200) {
       pagelog(this.responseText);
        }
     };
   req.send();
 }
 
 document.addEventListener('DOMContentLoaded', makerequest, false);
 </script>
@@ skipping 13 matching lines @@
       <div id="testcase-output"></div>
       <div id="testcase-output-expected">
         Text loaded via XMLHTTPRequest
       </div>
     </div>
     <div class="testcase-row">
       <h3>Image</h3><img class="blocked" src="/testcasefiles/xmlhttprequestexception/image.jpg"/>
     </div>
   </div>
   <h3>Filters</h3>
-  <pre>||testpages.adblockplus.org/testcasefiles/xmlhttprequestexception/*</pre>
-  <pre>@@testpages.adblockplus.org/testcasefiles/xmlhttprequestexception/$xmlhttprequest</pre>
+  <pre>||{{ site_url|strip_proto }}/testcasefiles/xmlhttprequestexception/*</pre>
+  <pre>@@{{ site_url|strip_proto }}/testcasefiles/xmlhttprequestexception/$xmlhttprequest</pre>
 </section>