All test subtargets fail due to import error

[eloquence]

The Makefile provides several convenient sub-targets: test-proxy, test-whonix, test-gpg, test-app, test-base. However, they all fail with:

ImportError: No module named 'base'

[eloquence]

As far as I can tell, to fix this, we can either use the discover subcommand throughout, e.g.

python3 -m unittest discover tests/ -v -p test_proxy_vm*

Or we can alter the import logic in the tests themselves. Using discover throughout seems like the simpler fix to me.