Make it possible to run utilities tests with --watch

Hey! I'm starting to play around with possibilities for an `overwriteChainableMethod` utility to address issue #215, and I was trying to run the utilities tests with:

```bash
mocha --watch --require ./test/bootstrap --reporter spec test/utilities.js
```

But the tests fail if they run more than once because they expect certain methods to not be part of the `Assertion` prototype. Would it be okay for me to add some cleanup in an `after()` block?