Add `toHaveScopes` helper method

Allows checking if an array of expected scopes are present. Refs [a comment of mine in #16](https://github.com/jugglingnutcase/language-powershell/pull/16#issuecomment-55613267).

Mind checking my work @jagrem? i just wrote it up myself, it was my idea, so i felt bad asking you to code it (esp since it's not a big addition).