Move tests for DETAILS and SUMMARY elements to html/details_summary/.

third_party/WebKit/LayoutTests/html/details_summary/details-position.html

 <style>
 body { overflow: hidden; }
 
 summary.fixed {
     position: fixed;
     left: 50px;
     top: 150px;
 }
 
 summary.relative {
@@ skipping 17 matching lines @@
     </details>
     <details>
         <summary class="relative">relative</summary>
         <input>
     </details>
     <details>
         <summary class="absolute">absolute</summary>
         <input>
     </details>
 </body>