Basic Blender mock for unit testing

[Mimickal]

We need a fixture we can control in tests that emulate the data we'd get from Blender's API. This will allow us to write unit tests that don't depend on a real instance of Blender.

This task is just to get the groundwork in place to allow our "production" code to transparently switch between the mock and the real Blender API when running tests.

[Mimickal]

Could potentially wrap / extend Blender's API, so we can call the real functions when needed?