Add example makefiles that are violating rules (one file for each rule + one without violations)

[christian-weiss]

Would help to test if checkmake still works (after refactoring) and also to illustrate how each rule works.

[mrtazz]

This is a great idea. There are some example makefiles in https://github.com/mrtazz/checkmake/tree/master/fixtures that are being used for tests. But not all rule tests use a fixture like that. Any addition there would be great.