Simplify unit tests for Evolver.
Simplify unit tests for Evolver.

There's a lot of state in `Evolver` that the unit tests need to check,
and this led to some verbose assertions, which would soon become more
verbose.

To keep things manageable, this change makes use of some new unit test
additions for checking applied evolutions, adds a new method for
checking migration plans, and updates stored migration plan state from
signal emissions to help assert the contents.

Testing Done:
All unit tests pass.

Reviewed at https://reviews.reviewboard.org/r/11258/

beanbaginc/django-evolution