Remove migration spec that uses a global model.

This migration spec creates a global model reference, but the entire
migration isn't really needed.  The spec isn't testing anything except
pure ActiveMigration code, which isn't worth doing.
Fix issue where migration spec assumes AwesomeSpawn exists, but doesn't.
Changed migrations specs to require that migration_stub calls exist.

Before, using migration_stub could reference a global model.  The goal
of migration_stub always was to prevent loading global models so that
we can ensure that, at the least, tests enforced that rule.  Now,
migration_stub actually enforces the rule.

This exposed many places where migration_stub was being used incorrectly
or where the migration itself incorrectly referenced global models.  The
fixes, depending on the migration, include
- Add the stub if it's used by the migration itself, and was never there.
- If the stub is not used by the migration, but only for testing, add
  a spec-only `Class.new` style stub.  Most of the `Reserve` specs do
  this, so, a helper method was introduced to DRY it up.
- One migration specs used stubs, but upon further review that spec
  wasn't testing anything except pure ActiveMigration code, which isn't
  worth doing, so it was removed.