Try to install Salt 3006.8 as test dependency

Summary:
To install Salt is a complicated story with the migration outside
the regular Python packages system to use "one directory" style.

With Salt itself, and the main dependencies, we have success in
some scenarii to install Salt and be able to run tests.

A known issue is Salt can't load its own dependencies.

Reviewers: yousra, Duranzed

Reviewed By: yousra, Duranzed

Differential Revision: https://devcentral.nasqueron.org/D4001