ZipStore tests may not identify bugs involving get and set

[maxrjones]

The set and get methods for StoreTests should be independently implemented but the ZipStore tests use the store methods, meaning that the behavior will be tested against itself rather than the expected behavior. This issue is to decide if the get and set methods for TestZipStore should be changed to not use ZipStore methods.

[d-v-b]

good catch, I would recommend implementing separate test-suite-only get and set methods, as per the other stores