Made deserialize_db_from_string() do not sort dependencies.
deserialize_db_from_string() doesn't use natural keys, so there is no
need to sort dependencies in serialize_db_to_string(). Moreover,
sorting models cause issues for circular dependencies.
deserialize_db_from_string() loads the full serialized database
contents, which might contain forward references and cycles. That
caused IntegrityError because constraints were checked immediately.
Now, it loads data in a transaction with constraint checks deferred
until the end of the transaction.
Previously, this test could modify global state by changing
connection.settings_dict. This dict is a reference to the same dict as
django.db.connections.databases['default'], which is thus also changed.
The cleanup of this test would replace connection.settings_dic` with a
saved copy, which would leave the dict itself modified.
Additionally, create_test_db() would also modify these same dicts, as
well as settings.databases['default']['NAME'] by adding a "test_"
prefix, which is what can cause problems later.
This patch:
- makes a complete copy of the connection and work on that, to improve
isolation.
- calls destroy_test_db() to let that code clean up anything done by
create_test_db().
