Fixed test for models in non-installed apps.

Models are now attached to any application they're defined in. Since
not_installed was inside app_loading, these models were mistakenly
attached to app_loading. The test that used them passed accidentally
when run after EggLoadingTest because that class' tearDown() method
replaces apps.all_models['app_loading'] by a copy of itself, while
it should remain the same as apps.app_configs['app_loading'].models.

tests/app_loading/not_installed/models.py

@@ -2,12 +2,22 @@
 
 
 class NotInstalledModel(models.Model):
-    pass
+
+    class Meta:
+        app_label = 'not_installed'
 
 
 class RelatedModel(models.Model):
+
+    class Meta:
+        app_label = 'not_installed'
+
     not_installed = models.ForeignKey(NotInstalledModel)
 
 
 class M2MRelatedModel(models.Model):
+
+    class Meta:
+        app_label = 'not_installed'
+
     not_installed = models.ManyToManyField(NotInstalledModel)