add textarea to prettify_test.

Author: mikesamuel@gmail.com

tests/prettify_test.html

@@ -1297,6 +1297,9 @@ <h1>Go mode</h1>
 
 /* " */  "foo /* "  /*/  */
 </pre>
+
+Test IE by copy/pasting content here.
+<textarea cols="80" rows="40"></textarea>
 </body>
 
 <script type="text/javascript">