Tests that client can call undo multiple times with non-empty history.

========= Original ========
  <div style="display:none" id="container">\n</div>
===== Modified element =====
  <div style="display:none" id="container" foo="bar">\n</div>
===== Undo 1 =====
  <div style="display:none" id="container">\n</div>
===== Undo 2 =====
  <div style="display:none" id="container">\n</div>