Add marked as skipped

I reckon I've probably forgotten something here. But this is the basics of adding skipping tests.
Ah. A fair point. I suppose this is a difference between an xUnit style, and a BDD style used by peridot.

As you said - exceptions seem to be a pretty [standard](https://github.com/sebastianbergmann/phpunit/blob/master/src/Framework/Assert.php#L3064) practice.

I will merge! Thanks for your work @icambridge