add support for `this.sinon` in `after` and  `before` hooks

not sure if this creates complex cases in suites that use combinations of all of the before/after/beforeEach/afterEach, but for my simple case it seems to work OK. I guess with both `before` and `beforeEach` you'd create an unnecessary sandbox in `before` that just gets replaced by the ones created in `beforeEach`, but I don't think that's a big deal. This does let you use just `before` and `after` though, which was my motivation for this PR.