Move some test helper functions into TestCase, and improve testability.

This improves the test capabilities a bit by taking some useful
functions from `EvolutionTestCase` (`assertSQLMappingEqual()`,
`get_sql_mapping()`, and related attriubtes) into `TestCase`, so that
more unit tests can have access to them.

New utility and assertion functions have been added for unit tests to
use. These include:

* `record_evolutions()`: Records new `Evolution` entries in the
  database.

* `record_applied_migrations()`: Records new applied migrations in the
  database.

* `assertEvolutionsEqual()`: Asserts that a list of `Evolution` entries
  match expectations.

* `assertAppliedEvolutions()`: Asserts that a list of applied
  `Evolution` entries match expectations.

* `assertAppliedMigrations()`: Asserts that a list of applied migrations
   match expectations.

And finally, upon teardown of a unit test, the built-in test apps are
now deleted from the database and unregistered from Django.

Testing Done:
Unit tests pass on all versions of Python.

Reviewed at https://reviews.reviewboard.org/r/11257/