Add tests for downloading packages with ssh

[Kroppeb]

I've noticed there is a marker called needs_github_ssh in the test suite, but no tests seem to have this marker, and even if there were such tests, it seems like tests with this marker would always be skipped.

[matteius]

When I started working on pipenv there were possibly some tests that were always skipped because the issue of provisioning a password protected ssh key and protecting it for the CI to use for ssh and for it to have access to some repositories to test against. This is certainly non-trivial to get ssh tests into the CI.