This modernizes python code without breaking Py2 compat.

Ran with command:

futurize --stage1 -w tools/testrunner

Bug: v8:9871
Change-Id: Ie23333cbd923197be0bffcad5041056e00990042
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2252554
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Tamer Tas <tmrts@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68496}