Update //agents presubmits to support absolute imports

Updates the PRESUBMIT.py files under //agents to set a PYTHONPATH when
running unittests. This allows code to use absolute imports without
having to modify PYTHONPATH directly. The switch to absolute imports
will come in follow-up CLs in order to fix errors found by pytype while
also better adhering to PEP8's guidelines.

Bug: b/528058003
Change-Id: I93910718cc8db301ca75844bea4e84fa1a9fbe33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8013148
Auto-Submit: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Jonathan Lee <jonathanjlee@google.com>
Reviewed-by: Jonathan Lee <jonathanjlee@google.com>
Cr-Commit-Position: refs/heads/main@{#1653520}