Tests were failing because of:
* Models weren't being imported from their new location.
* PolymorphicManager doesn't work with non-polymorphic models, but
migrations run models' manager, so having MROBase3 with
PolymorphicManager failed.
* Some tests failed due to have assertions were being done (since they
hardcoded expected instance ids). Update tests to use comparisons
similar to #312
e102a0502c