- Remove all relative imports from mock and os
- Fix mocking in a few tests to prevent cross-test side-effects
- Add run_perf_test.py to v8_presubmit.py
- The vpython config was not added since root .vpython already includes
  coverage and mock libraries
- Convert all double-quoted strings to single-quoted (PS8->PS9)

R=sergiyb@chromium.org

Bug: chromium:123456
Change-Id: I7b3a08dc5d950b0f51cc7a5eb3a012ea953ca824
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1564206
Commit-Queue: Sergiy Belozorov <sergiyb@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60810}