Move subcommand_test to py3
Change-Id: Iabc6ceb75da8a71f089c8fd89c94890f7b3f428e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2106513
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>