Adds tests using packse test scenarios! Uses `test.pypi.org` as a
backing index.
Tests are generated by a simple Python script. Requires
https://github.com/zanieb/packse/pull/49.
This opens us to a slight attack surface, as we cannot force use of
`test.pypi.org` only and someone could register these package names on
the real `pypi.org` index with malicious content. I could publish these
packages there too.
Zanie Blue