Add unit tests for PuppetLint::Checks

[rodjek]

No description provided.

[rnelson0]

It looks okay to me, but I'm not sure I understand the purpose of this change, can you elaborate?

[rodjek]

The change is adding unit tests for the PuppetLint::Checks class, which is not currently explicitly tested. Some parts of it were implicitly tested by the check plugin tests themselves, but not all of it. Adding tests like this gives visibility to unexpected behaviour changes that might occur as a result of future work.