Don't run store tests without a store!

Store tests reuse a base test case which defines relevant test
functions. However, the base test case should never run independently,
only in a subclass, because it has no store to test.

Somehow, this issue only occurs if you run tox -- --tests
glance/tests/functional/store

Change-Id: I5d8d26ba68d034fb8e94e5408a757d9869825285
Merge "Don't run store tests without a store!"