MIT: CCX. Fixes for API changes that affect the binding of descriptors to student context.
MIT: CCX. Fixes for API changes that affect the binding of descriptors to student context.

fix for making sure all descriptor fields are always available for course descriptors despite module_class tricks

add required fields to cache data for tests

use a different strategy to mock up the get_children method

bmedx/modulestore,bmedx/modulestore,bmedx/modulestore