Adds tests for assignment to and deletion from 1:n ForeignFields
Adds test for assignment by index of a ForeignList
Adds test for the "in" operator on ForeignList fields
Adds a (failing) test for inserting a new object into a ForeignList field.
Adds (failing) test for removing an object from a ForeignList via the .remove() method.
Removes __init__.py - subdirectories of /tests/ need not be modules.