[client] kill process group to prevent process leak
[client] kill process group to prevent process leak

This is similar to https://crrev.com/c/2417650 but for run_isolated.py.

Bug: 1122787, 1073832
Change-Id: Ia6479ed53f17ae1e6935c4fc6a64ccde1bc3ad4d
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2437654
Commit-Queue: Ye Kuang <1cd89f22f3967fc3dcb883b53940be4b98ec483f@google.com>
Reviewed-by: Ye Kuang <1cd89f22f3967fc3dcb883b53940be4b98ec483f@google.com>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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