For an upcoming refactoring of library remapping we want to be able to test if the logic won't change.
It also increased my experience inside the remapping codebase and find out what exactly needed to
be refactored.
This patch adds test cases for the core functionality of `foreach_libblock_remap_callback`. The test cases
don't cover of all the branches. Also pre-, post-processing, referencing and proxies are not tested.
Reviewed By: mont29
Differential Revision: https://developer.blender.org/D13815
