Added a first test case for review. This will be the base for future test cases.
The current API is sufficient for what is expected for such a low level API.
One concern is that you need to trigger a save in order to update the library overrides
structure. Not expected from TD/Dev point of view.
Test cases are very important when implementing restrictive mode as it is a second evaluation mode that
has impact on the (current) permissive mode.
Reviewed By: Sebastián Barschkis, Bastien Montagne
Differential Revision: https://developer.blender.org/D10747
