Add more tests for util functions

[rachidlamouri]

• Installed cross-env since I develop on windows, otherwise setting env vars in the npm scripts doesn't work
• Added a lint config for the tests
• I think some of the logic for validate is wrong. I left a TODO in the tests to discuss that
• I don't understand the else in clean. It recursively calls clean on the object's property, but it passes the same schema? Shouldn't it pass the schema for that property?
• IMO some of these functions would be easier to unit test if all the utils were attached to a single object. That way when I'm testing a recursive function I can assert that it calls itself for certain test cases which will limit the total number of test cases. Or I can assert that it calls another utility function which I have already tested.

[pateketrueke]

Awesome, thank you!

[rachidlamouri]

@pateketrueke If you look in PR description there were a couple of bullets I was hoping you would address. Mainly the validate function having some unexpected behavior as well as the behavior of the clean function

[pateketrueke]

Can you open an additional issue to address them? Thank you!