Allow installing CircuitBreakerStorageBasedTestCase as a pip extra

[jacobg]

I wrote a custom CircuitBreakerStorage class for a different cache backend than Redis, and I'd like to test it using CircuitBreakerStorageBasedTestCase. But the tests module is not included in setup. Please allow installing CircuitBreakerStorageBasedTestCase as a pip extra, preferably separate from the rest of the tests stuff.