Also enable all config/tests with python3. unit-nsinstall.py was the
last one that didn't pass with python 3.
Switch the test to using @unittest.skipIf and disable the subprocess
test because we purposely broke running nsinstall.py independently with
python 2.
Differential Revision: https://phabricator.services.mozilla.com/D70160
--HG--
extra : moz-landing-system : lando