The mock is designed to mimick a native call to `navigator.mozContacts.find({ filterBy: [...], filterValue: '...', filterOp: 'contains' })`. I studied the semantics and reproduced them in order to document the correct and expected behaviour when matches are found *and* when matches are not found. I could've passed a `MockContact.list()` and be done with it, but that doesn't give me a way of asserting the "no match" case with arbitrary filterValue params and to do so, I needed some degree of basic filtering capabilities. 

That aside, I've remove the looping logic and made it return an empty array or the result of a call to `MockContact.list()` conditionally.