Rename some test modules

test_hookcaller.py:
 We're not testing "method ordering" per say, we're testing the
 general internal API and behaviour of the `_HookCaller`; its
 method ordering is the majority of its purpose, yes. There's also
 a bunch of tests in this module that don't belong.

test_invocations.py:
 You can't really test the "hookrelay" (unless you're trying to test
 `getattr`). Really this module is testing the different types of hook
 "invocations" in terms of API and execution behaviour.

Resolves #100

Author: Tyler Goodlet