Any sane platform would process foreground tasks in a FIFO order. Hence
our mock platform in the tests should do the same.

R=ahaas@chromium.org

Bug: v8:7921
Change-Id: Iafe1d9f2eb2a0c168f7e994f05b054756ef9e4af
Reviewed-on: https://chromium-review.googlesource.com/c/1319754
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57283}