This CL updates the wasm spec tests. In addition it adds an
assertNotEquals function to mjsunit.js, and it fixes the test harness
to not call quit() because it causes a dead-lock in combination with
async compilation.

R=rossberg@chromium.org

Change-Id: I50cf737993adb3e2bd27977efe7e20e304b89078
Reviewed-on: https://chromium-review.googlesource.com/558077Reviewed-by: Andreas Rossberg <rossberg@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46366}