Fix Travis tests

[mcgege]

The problem was maybe? introduced with Puppet 4.19, don't know ...

Travis problem

[schurzi]

I think I fixed that tests with #259 but there are several other issues:

• the test for fact retrieve_system_users should also include a user, which is not a system user and conecuently is filtered by our fact
• the test for fact home_users should not check for user names in result, but for directories.

Both issues have one underlying problem:
Our mocked users do not have a uid or a home directory. I tried several ways to inject these two attributes, but I had no success.