RabbitConsumerSpec uses mutable state in its corresponding Fixture

[rodoherty1]

The static analysis check (scala-style) by default prohibits mutable state in application code and unit tests. The "prohibit mutable state" rule was disabled during the hackathon to facilitate adding of some unit tests.

This rule should be reinstated.

Please refactor RabbitConsumerFixture so that mutable state is not required for the tests to pass.