Switch test_app generation over to use py.test fixtures.

Switch test_app generation over to use py.test fixtures.

By doing this, we can take advantage of py.test's ability to create
temporary directories that are then cleaned up later during testing.
This helps for sandboxing things.

This also involves a ton of changes:
 - Changing the get_app stuff appropriately, getting rid of the
   setup_fresh_app decorator
 - Making said fixture
 - Switching over a billion tests to use it