This adds two tests to verify that the --liftoff flag has the indented
effect, and that Liftoff compilation is off by default.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Ie7e13184b5068f572b78dbdf7abbcded6d859fc5
Reviewed-on: https://chromium-review.googlesource.com/733561
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48995}