Reland "[CodeHealth] Migrate remainder of testing/ to Python3 compatible
syntax"

This is a reland of commit 8defc6c6da91e4b7d7a686b7bb67658d9a5ebe55

Uses `sys.path.append(...)` to import the `common` module in
testing/scripts/run_variations_smoke_tests.py (as is done elsewhere in
the original CL) and resolve errors thrown for attempting a relative
import with no known parent package.

Original change's description:
>and Pylint2.7, and fix resulting errors.
>
>Bug: 1262363
>Change-Id: I09628427dd4caf9d873f3b489b897054430f8567
>Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3557490
>Reviewed-by: Ben Pastene <bpastene@chromium.org>
>Commit-Queue: Joshua Hood <jdh@chromium.org>
>Cr-Commit-Position: refs/heads/main@{#996200}

Bug: 1262363
Change-Id: I60fe062f62962d0b3dc4e93d9e29626ed181f688
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3610783
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Joshua Hood <jdh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#999432}
NOKEYCHECK=True
GitOrigin-RevId: 3fade1f647d4dc195095434b4f28ed173ee61f2b
2 files changed