assertPromiseResult caused tests to timeout when the result of the
promise was unexpected, e.g. rejected instead of the expected
fulfillment. This CL cleans up the implementation of
assertPromiseResult, adds better stack traces, and adds tests for all
the important cases I can think of.

R=mathias@chromium.org
CC=clemensh@chromium.org

Bug: v8:7570
Change-Id: I6ecb94fd3e5151502edf73c3bcdeb518b80fc81c
Reviewed-on: https://chromium-review.googlesource.com/1032786
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52882}