Bug 1070845-r1 - [sharedtest] unit test refactoring for new version chai.assert api

I find this pretty confusing, mostly because it is not intuitive to me that handling a rejected promise results in a resolved promise. I'd change the name from check() to ensureReject() or something.  And I'd add a comment somehow indicating that you're wrapping the promise in another promise that is going to invert its state.