Also, I'm fairly certain `sinon.mock` doesn't work with constructor methods... Perhaps a stub or spy, but since the purpose of this is just to provide a replacement for `ManifestHelper` that does minimal work to still act like a `ManifestHelper`

I think that the way I've done it is the pattern that has been done all across gaia. You can tell that this was code that was here well before I got to this file, I just moved it into `shared/test/` added `'use strict';` and turned it into a named function.