This reverts commit 91023d79 as it
increases memory usage for the test suite.

Thanks Tim Graham for the review.

TestCase subclasses are wrapped in a transaction that prevents any data from
being persisted between test runs andi thus don't require limiting the tables
to be flushed to a subset of available apps like TransactionTestCase subclasses
do.

Also removed try/except/fail antipattern that hides exceptions.

Also renamed app_cache to apps and "app cache" to "app registry".

Deprecated AppCache.app_cache_ready() in favor of Apps.ready().

Invalidate properly the cache whenever all_models or app_configs change.
This fixes some isolation issues in the test suite.

Reverted 4a56a93c.

Since the original ones in django.db.models.loading were kept only for
backwards compatibility, there's no need to recreate them. However, many
internals of Django still relied on them.

They were also imported in django.db.models. They never appear in the
documentation, except a quick mention of get_models and get_app in the
1.2 release notes to document an edge case in GIS. I don't think that
makes them a public API.

This commit doesn't change the overall amount of global state but
clarifies that it's tied to the app_cache object instead of hiding it
behind half a dozen functions.

This commit doesn't contain any code changes; it's purely a refactoring.

Should be unneeded with Python 2.7 and up.
Added some unicode_literals along the way.

By avoiding to run syncdb with the full set of test models.

Thanks Anssi for the idea.

This is necessary because get_model() checks are case insensitive, and if the swapable check isn't, the
swappable logic gets tied up in knots with models that are partially swapped out.

Thanks to chris@cogdon.org for the report and extensive analysis, and Preston for his work on the draft patch.

This is required to allow the test to run without error under SQLite in the CI environment.

Thanks to rizumu for the report.