Refactored the createSpy tests, to add unit tests for its part of call tracking

Currently, the callTracker functionality is tested in great detail in CallTrackerSpec, but createSpy is only tested as part of one large test in spec/core/EnvSpec, which tests the whole bunch in one go, and that's really an integration test anyway, and is only asserting on CallTracker methods.

This patch:

* Creates some unit tests for call tracking in createSpy itself
* Moves the integration testing code from spec/core/EnvSpec to a new test in spec/core/**integration**/EnvSpec, with the other integration tests
* Changes the original EnvSpec test to now only test what it should be (according to its name): that it 'overrides the method on the object and returns the spy', rather than actually then testing the spy functionality