The environment variables for swarming shards are leaking into the gtest runs, which read them as well and in turn skip some tests. Now we make sure those environment variables aren't passed to the subprocesses. 

BUG=v8:5956

Change-Id: I9c93b1facc703a10a88e633074977743ccd24eb0
Reviewed-on: https://chromium-review.googlesource.com/441745Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43161}