fixtures: in-memory queues are not cleared

[mdonadoni]

if in-memory queues are not cleared tests fail.

The test fails for both MessageConsumer and scheduler. This is probably due to in_memory_queue_connection having a "session" scope. It means some messages can stay in the queue between tests and cause unexpected failures.

As for now, I solved it by cleaning messages at the end of each test. But, this is not a sustainable solution. It is not nice when you need to remember to clear stuff after the test. I propose to create an issue to address later.

Originally posted by @VMois in reanahub/reana-server#487 (comment)