More createMockEditor() cleanup

I found some more places in the unit tests where calls to createMockEditor() did not have matching calls to destroyMockEditor(). You can see evidence of this by doing this:

1. Debug Run > Tests
2. Run CSSUtils tests
3. Run Live Development tests

Results
Notice that Spec Runner window now has a scrollbar that indicates the window grew in height. Scroll down to see Dom objects left behind. Or open Dev Tools Elements inspector to see the <div id="mock-editor-holder"> objects.