Add a base test class for the queues package

Tests for `Messages` and `Claims` will inherit from this new base class
instead of inheriting from the QueueTestBase.

Change-Id: Iddd7d3f538c158f8cf9d8e08654fa524e69e6622
Partially-Implements: python-marconiclient-v1
Merge "Add a base test class for the queues package"