Investigate mock for better unit tests

[gdevenyi]

http://slviki.com/index.php/2016/06/18/introduction-to-mocking-in-python/

[gdevenyi]

Related to #121

[pipitone]

Given @dcoynel's pain in getting slurm going (see #140), and given that our tests don't actually depend on any of the scheduling systems to be installed right now (since we never schedule any jobs in our tests), perhaps one concrete thing we can do is mock the scheduler system commands (see testpath, for instance).