[subprocess42] Add test case for leaked child.
[subprocess42] Add test case for leaked child.

This was more annoying than it should have been, but I think this test
concretely shows that we can kill off leaked subprocesses (even after
wait()).

Requested in https://chromium-review.googlesource.com/c/2416410

R=jwata, maruel, tikuta, vadimsh

Bug: 1127089
Change-Id: I0ea11f3a42c69e446e9fb8d02077d248349c3f03
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2425120
Auto-Submit: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py