test cases per discussion in D7923362
test cases per discussion in D7923362

Summary:
Discussion in D7923362 included validity & behavior of `wait()` vs. fulfillment and `wait()` vs. `then()`.

18 test-cases:

8 cases with `.wait(...)` and fulfillment:

- Future vs. SemiFuture
- wait() vs. wait(dur)
- fulfilled before wait(...) call vs. during wait(...) call

10 cases with `.wait(...)` and `.then(...)`:

- 4 wait() cases: {Future vs. SemiFuture} that gets fulfilled {prior to, during} the wait() call.
- 6 wait(dur) cases: {Future vs. SemiFuture} that gets fulfilled {prior to, during, after} the wait(dur) call.

Reviewed By: yfeldblum

Differential Revision: D8048426

fbshipit-source-id: c79cc337706f08f8fb9c6057a114723320a90820

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly