Add unit test for salt.utils.which

This is a Linux-only test, it will need some mock love to make it
suitable to test for windows paths.
Provide a mocked version the tests to reach windows specific code.
Add some comments to explain whats going on.
Merge branch 'features/mocked-which-test' of https://github.com/s0undt3ch/salt into s0undt3ch-features/mocked-which-test