Deferred accessing the module-global variable __file__ because the
Python import API does not guarantee it always exists—in particular, it
does not exist in certain "frozen" environments. The following changes
advanced this goal.
Thanks to Carlton Gibson, Tom Forbes, Mariusz Felisiak, and Shreyas
Ravi for review and feedback.
Added comments in the three empty models.py files that are still needed.
Adjusted the test runner to add applications corresponding to test
labels to INSTALLED_APPS even when they don't have a models module.