Circular dependence to bob.blitz during testing

This package depends on bob.blitz to run its test units. This is a problem in test environments (like conda packaging) where package downloading via setuptools is forbidden. We need to fix these tests by creating units which independent from other packages entirely. For the time being, such tests are skipped when bob.extension's conda package is built with --exclude=test_extensions. See logs with problem at bob/bob.conda!62.