It has been only used in the builtin_servers tests since Django 1.4.

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.

Refs #20680.

Thanks to amosonn at yahoo.com for the report, @doda for the initial patch and
@datagrok for the revamped logic and test case.