Issue 4687 - Add Context Function get_pages_metadata to Test Site

tests/test_site/pages/sitemap.tmpl

 template = empty 
 product = foo
 title = test_metadata
 tags = [popular, bar]
 <h1> Unfiltered </h1>
 <ul>
     {%- for unfiltered_page in get_pages_metadata() -%}
         <li>title : {{ unfiltered_page['page']}} </li>
 {%- endfor %}
 </ul>
 
 <h1> Filtered </h1>
 <ul>
     {%- for filtered_page in get_pages_metadata({'tags': ['popular', 'bar']}) -%}
         <li>title : {{ filtered_page['page'] }} </li>
 {%- endfor %}
 </ul>
+
+<h1> String Filter For Listy Option </h1>
+<ul>
+    {%- for filtered_page in get_pages_metadata({'tags': 'popular'}) -%}
+        <li>title : {{ filtered_page['page'] }} </li>
+{%- endfor %}
+</ul>