Add efficient OCMock based test cases for `enqueueBatchOfHTTPRequestOperations`

Since @OliverLetterer isn't comfortable adding these tests to his PR since he doesn't (yet) speak OCMock, I thought I'd shoot them over.

These tests run much more quickly than their network backed brethren and better isolate the behavior under test. Plus they serve as a great reference for how to use OCMock to put very specific behaviors under the microscope for testing.