The notifications test suite was rather large, and is about to become

increasingly so. It was harder to run subsets of tests and to add new

ones in an organized way.
This change splits up the test suites into multiple modules. Some are

class-based, while others are module-based or responsibility-based. For

instance, since there's so much that ties into e-mail sending, there's a

tests module about e-mail sending, rather than one for each signal or

function.

Unit tests all pass.