Run unit tests only on commit since it's way too slow for upload. User
can still run tests on local machine via `git cl presubmit` or directly.

This CL reduces time it takes to upload from 2+ minutes to ~10s.

R=apolito@google.com

Fixed: 1263688
Change-Id: Id96c049296eea7c0a4d1e484dce6ae4296aa8041
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3249192
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Anthony Polito <apolito@google.com>
Reviewed-by: Anthony Polito <apolito@google.com>