test(alert): add more coverage and other minor changes

3 proposed changes:

There are tests for classes and close buttons, but the content itself is not tested. We need to be sure that the content is there. For consistency, I added a method and made a for loop like the other on the test (which I find a little bit redundant). I find by span (interpolations are converted to span). I would prefer to add a class to the transclude div, but that is not my decision.

The test that tests the classes. It test the first two and then some defaults.

Im my opinion, it should test that the third one (which doesn't have a type associated) should contain an `alert-warning` class.

On the other hand, I deleted the other expectations. All of them contains `alert` so I don't see the need to test that and if you want to be sure that your alerts only have one type and no others, that could be tested in another `it`.

Last, I removed a redundant "it".

Feel free to modify this or reject it at all :P