Add run_on_python2 and run_on_python3 to allow to be tests on
either only python2, only python3 or both.

Bug: 984182
Change-Id: I87d3b67412129cf3a8e627cd6bc97daa81147657
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1758849
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: smut <smut@google.com>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>