test(dropdownToggle): Test for disabled element

I am not that familiarized with bootstrap to be honest, but I saw that we added a fix without a test, so I added the test.

On the other hand, seems that well, the toggle won't work if we add the `disabled` class to the anchor (as expected), but the disabled attribute doesn't exist for the anchor and AFAIK, this directive is to being used only on an anchor. If so, that check can be removed from the code.