Issue 6769 - Avoid hardcoding the URL

pages/filters/xmlhttprequest.tmpl

 title = $xmlhttprequest
 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/xmlhttprequest/text.txt", true);
+  req.open("GET", "{{ site_url }}/testcasefiles/xmlhttprequest/text.txt", true);
   req.onload = function(e) {
     if (this.status == 200) {
       pagelog(this.responseText);
        }
     };
   req.send();
 }
 
 document.addEventListener('DOMContentLoaded', makerequest, false);
 </script>
@@ skipping 11 matching lines @@
     <div class="testcase-row">
       <h3>XMLHTTPRequest</h3>
       <div id="testcase-output"></div>
       <div id="testcase-output-expected">XMLHTTPRequest failed.</div>
     </div>
     <div class="testcase-row">
       <h3>Image</h3><img class="testcase-image" src="/testcasefiles/xmlhttprequest/image.jpg"/>
     </div>
   </div>
   <h3>Filters</h3>
-  <pre>testpages.adblockplus.org/testcasefiles/xmlhttprequest/$xmlhttprequest</pre>
+  <pre>{{ site_url|strip_proto }}/testcasefiles/xmlhttprequest/$xmlhttprequest</pre>
 </section>