Bug 916956 - Sinon sandbox not available in suiteSetup

I've named the suite's sandbox `suiteSandbox`. It might make sense to *also* define it as `this.sinon` because (although not as accurate) it will make usage of the sandbox identical from within `suiteSetup`s, `setup`s, and `test`s. I'm not sure if that could cause confusion, though: code that assumes `this.sinon` refers to the same object in `suiteSetup` and `setup` functions will not work. Will this detail actually matter?