Test isolation errors on master.

[Florian Apolloner]

./runtests.py select_for_update swappable_models --settings=test_sqlite
Creating test database for alias 'default'...
Creating test database for alias 'other'...
.Essssssss
======================================================================
ERROR: test_generated_data (swappable_models.tests.SwappableModelTests)
Permissions and content types are not created for a swapped model
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Traceback (most recent call last):
  File "/home/florian/sources/django.git/django/test/utils.py", line 220, in inner
    return test_func(*args, **kwargs)
  File "/home/florian/sources/django.git/tests/swappable_models/tests.py", line 39, in test_generated_data
    ContentType.objects.filter(app_label='swappable_models').delete()
  File "/home/florian/sources/django.git/django/db/models/query.py", line 558, in delete
    collector.delete()
  File "/home/florian/sources/django.git/django/db/models/deletion.py", line 262, in delete
    qs._raw_delete(using=self.using)
  File "/home/florian/sources/django.git/django/db/models/query.py", line 570, in _raw_delete
    sql.DeleteQuery(self.model).delete_qs(self, using)
  File "/home/florian/sources/django.git/django/db/models/sql/subqueries.py", line 86, in delete_qs
    self.get_compiler(using).execute_sql(None)
  File "/home/florian/sources/django.git/django/db/models/sql/compiler.py", line 763, in execute_sql
    cursor.execute(sql, params)
  File "/home/florian/sources/django.git/django/db/utils.py", line 101, in inner
    return func(*args, **kwargs)
  File "/home/florian/sources/django.git/django/db/utils.py", line 94, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File "/home/florian/sources/django.git/django/db/utils.py", line 101, in inner
    return func(*args, **kwargs)
  File "/home/florian/sources/django.git/django/db/backends/sqlite3/base.py", line 476, in execute
    return Database.Cursor.execute(self, query, params)
OperationalError: no such table: multiple_database_review

----------------------------------------------------------------------
Ran 10 tests in 0.046s

FAILED (errors=1, skipped=8)
Destroying test database for alias 'default'...
Destroying test database for alias 'other'...

This is caused by #15903 -- I don't see an obvious fix though, some stuff requires all models to be registered (tests come to mind where you change INSTALLED_APPS on demand, but the related cache is only filled once) while other stuff (like the collector) should never see models from not installed applications.