The hooks.py and testcases.py modules define a number of classes that don't have anything to do with each other except that they derive from the same base class. We should create packages for hooks/ and testcases/ to more easily organize the growing files.

In addition to splitting the hooks.py and testcases.py modules into separate files, we should also make it possible to introduce a new CustomBehavior class or a new TestCase class without having to explicitly add it to the dictionary of known classes.