This way, we can also check the return code of d8. We currently have a
bug (6981) which makes failing tests not being detected, even though
the failure message is (sometimes) being printed.
After this refactoring, we can write tests for our mjsunit test
functions.

R=machenbach@chromium.org

Bug: v8:6981
Change-Id: I0aa0abcb0f9a4f622a1e1d1a4d826da1e6eb4f07
Reviewed-on: https://chromium-review.googlesource.com/737991Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48951}