Call parent tearDown method to allow additional extension

Users may want to adjust the `setUp` and `tearDown` steps of their test suite. In the current arrangement, calling `parent::setUp` is required as it needs to make it's way up into Laravel but it allows allows users to customise the set up in their `TestCase`. Adding a parent call for `tearDown` allows users to customise the tear down procedure as well.

I have a possible use case for this - my current test suite often causes a MySQL "too many connections" error when run. If I call `DB::disconnect()` in my `tearDown` I'm able to prevent this from happening. While I don't think it's worth hard-coding this into `DbTestCase` it's important to provide users the option.