Refactors WLXServiceManagerSpec beforeEach block.

Initializes all mocks first in order to avoid sending
messages that haven't been stubbed yet.
Replaces NULL for DISPATCH_QUEUE_SERIAL when creating queues.

The attr parameter of the dispatch_queue_create must be set
to DISPATCH_QUEUE_SERIAL when creating a serial queue.
Although the actual result is the same (because
DISPATCH_QUEUE_SERIAL is defined as NULL) it is clearer
to use a named constant instead.