Enable import-error check on //testing

Enables the import-error pylint check on //testing. Most errors are
fixed by updating the PYTHONPATH used by pylint to include the necessary
directories, although some import errors are simply suppressed (most
commonly for vpython-provided modules).

Also takes this opportunity to add comments about where each block of
non-standard imports comes from. This was already present in a number of
places, but not everywhere.

Bug: 353942917
Change-Id: I506d35e5a6ae56471618e6b4af57dd2ecb64dd7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6092016
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Paul Semel <paulsemel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1396194}
NOKEYCHECK=True
GitOrigin-RevId: fe2702e96ce66e27b4e474f4a39eb78813de5fcc

• exe_util_unittests.py[diff]
• generate_script_unittests.py[diff]
• main_program_unittests.py[diff]
• rust_main_program_unittests.py[diff]
• test_filtering_unittests.py[diff]