This replaces multiprocessing with threading in tests to make python
coverage work.

TBR=sergiyb@chromium.org

Bug: v8:6917
Change-Id: Idff763dfefa4a7fc782133d94089b3a5b00a194d
Reviewed-on: https://chromium-review.googlesource.com/844735Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50344}