Test back button

[codeguru42]

Test back button for correct behavior. This is separate from #333 which implements (and tests) the up button on the action bar.

[codeguru42]

Some of these tests already exist:

1. BaseballCardListWithDataTest.testAddCardToPopulatedDatabase()
2. BaseballCardListWithoutDataTest.testAddCardToEmptyDatabase()

Several other tests in BaseballCardListWithDataTest also click the back button when on the Filter Cards screen, but these are actually testing for other things as well. I think it would be helpful to have a test specifically for the back button in this situation as well as test the back button from the About screen.