Issue 6769 - Avoid hardcoding the URL

pages/filters/stylesheet.tmpl

 title = $stylesheet
 template = testcase
 
 {% set testcase_moreinfo = [
   ("Filter Options", "https://adblockplus.org/filters#options"),
 ] %}
 
 <link rel="stylesheet" href="/testcasefiles/stylesheet/stylesheet.css">
 
 <section class="site-panel">
   <h2>$stylesheet</h2>
   <p>Check that usage of the $stylesheet filter option works as expected.</p>
   <p>With the filter displayed below each test case added to ABP (or with the testcase subscription installed and active), the stylesheets in each test case panel should be blocked but the images should not.</p>
 </section>
 
 <section class="site-panel">
   <h2>Test case</h2>
   <p>The stylesheet should be blocked. The image should not be blocked</p>
   <div class="testcase-container">
     <div class="testcase-row">
       <h3>Stylesheet</h3><div class="testcase-teststylesheet trap">&nbsp;</div>
     </div>
     <div class="testcase-row">
       <h3>Image</h3><img class="testcase-image" src="/testcasefiles/stylesheet/image.jpg"/>
     </div>
   </div>
   <h3>Filters</h3>
-  <pre>testpages.adblockplus.org/testcasefiles/stylesheet/$stylesheet</pre>
+  <pre>{{ site_url|strip_proto }}/testcasefiles/stylesheet/$stylesheet</pre>
 </section>