Synchronous promise mock for tests. Implemented using Typescript so types are provided with the
library. Also, the mock class implements Promise interface so you can use it wherever a real
Promise is required.
Forked and inspired by @slavik57's promise-sync.
I felt that his original implementation is a bit outdated and, more importanty, it does not
implement Promise interface so it is painful to use in places where a real promise is required.
(See slavik57/promise-sync#15 for my original issue)