Write unit (not system) tests for the APT sub-package

[laurenmarietta]

It has been made clear by #172 that there is a need for functioning tests to maintain the APT sub-package. The existing test in apt_test.py is a system test that calls yaml_generator.py and thus needs a real value for the MIRAGE_DATA environment variable, so it cannot be used by the CI (see discussion in #156).

It should be a priority to generate smaller tests that can be run through the CI.