Add self validation for exercises

[asimell]

It would be nice if participants could run a ready script that validates they've done the exercise correctly.

[asimell]

Possible ideas:

• Have a "validation test suite" that resources the exercise resource file, calls a keyword and then validates that expected things happened. It should not got into the details of how said keyword was defined, but just the end result that it actually did what was expected. Downside: forces keywords to be named in exactly the same way so the validation test suite knows how to call them
• Have the participants add a specific line that is then parsed by "something" to be used in the validation
• Linter. Downsides: Requires quite a bit of work to make custom rules for all files and allow some freedom for keywords