Free memory allocated by testElement after usage

IE11 reports `testElement` as detached DOM node consuming memory. It is captured in closure of `setInnerHTML` function and could be safely cleared after feature test.

src/renderers/dom/client/utils/setInnerHTML.js

@@ -79,6 +79,7 @@ if (ExecutionEnvironment.canUseDOM) {
       }
     };
   }
+  testElement = null;
 }
 
 module.exports = setInnerHTML;