Add test to ensure that generated app runs and has 100% test coverage

[ssteinerx]

As per a note on the Django 1.10 release, pulling out into its own issue.

The tests passed, but the generated app wouldn't run on the 1.10 release.

Also, testing the claim of "generating a Django project that has 100% test coverage out of the box."

IOW, not cookiecutter-django itself, but the project generated therefrom.

So, as part of the tests, we generate a project, then ensure that it runs and has 100% test coverage.

[browniebroke]

I guess we could start by adding a line like this at the end of our test scripts: python -m coverage report --fail-under=100

Source: https://hynek.me/articles/ditch-codecov-python/