Clean up orphaned Dart processes on test.py shards on Windows

Fixes: https://github.com/dart-lang/sdk/issues/32138
Change-Id: I513316b33a84bcbc024ff589727bdfeaadd0fc99
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/170100
Reviewed-by: Karl Klose <karlklose@google.com>
7 files changed