build_speed: Add unit tests and refactor benchmark script

Add comprehensive unit tests for the benchmark.py script to improve
robustness and prevent regressions. A PRESUBMIT.py file is also added to
run these tests automatically.
- Avoided testing private methods and some methods entirely (e.g.
  _emulator) as they are more trouble than it's worth. Those can be
  tested when running the entire script.

Additionally, refactor the script:
- Simplify emulator architecture detection.
- Add a --test flag to swap the target to chrome_public_test_apk.
  - Since targets are per-run instead of per-test, this means that
    whenever --test is used the caller is responsible for choosing the
    appropriate suite.
- Add a --quiet flag to suppress summary output.

Also add hnakashima@ to help own this dir.

R=agrieve@chromium.org

Bug: 391374982
Change-Id: I8baf197745cb4734f2962a0862e27f703a97e1a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6930870
Auto-Submit: Peter Wen <wnwen@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1513174}