Add test for setting textContent of LI element

Adds a test case for setting and reading back the textContent of a LI element that is attached to the dom.

The dom attachment is significant as some browsers attempt to apply styles to the content when reading back the value. I recently reported this as a regression in IE 11 on Connect.
http://connect.microsoft.com/IE/feedback/details/805580/incorrect-textcontent-for-li-elements