[enterprise][e2e] Add `pylint` presubmit check for tests and infra

Add a `pylint` check to run on upload and CQ+{1,2}, fix some trivial
problems, and add a TODO to address the more difficult ones later.

The benefits are:
1. `git cl presubmit` can catch trivial mistakes like unused variables
   (e.g., https://crrev.com/c/6485907) without actually running the
   tests, which are slow compared to other Chromium test suites.
2. Eng teams that use Python only rarely will own more of the
   test-writing, so this change enforces a code health baseline
   mechanically.

The possible downsides are slow uploads and false positives, but most
CLs only change 1-2 files, and a comment can disable lints selectively
as needed.

Bug: 413421824
Test: git cl presubmit --files='chrome/test/enterprise/e2e/**/*.py' --upload --verbose --force
Change-Id: Ibbc24bd7a29e671278f23cc574a31633f04f965c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6505842
Reviewed-by: Xiang Ji <jxiang@google.com>
Commit-Queue: Xiang Ji <jxiang@google.com>
Cr-Commit-Position: refs/heads/main@{#1455247}