[roll pigweed] pw_unit_test: Cleanup test processes properly on timeout

When processes launched were killed due to timeout, their child
processes were orphaned and left running. Add a trap to the test runner
script to cleanup the test processes when terminated. Switch to using
SIGTERM instead of SIGKILL to allow the script to catch and handle it.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/107777

https://pigweed.googlesource.com/pigweed/pigweed
pigweed Rolled-Commits: 57456a4254640a7..963e9bd4d97ba22
Roller-URL: https://ci.chromium.org/b/8805101222798058017
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I69b91bee4d4cf43b28945dd3042e2f780073d3cf
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/zephyr-integration/+/107851
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/pigweed b/pigweed
index 57456a4..963e9bd 160000
--- a/pigweed
+++ b/pigweed
@@ -1 +1 @@
-Subproject commit 57456a4254640a7978fe8921084f070f20c607f8
+Subproject commit 963e9bd4d97ba227c4283090b58b8b706eceff5d