This is a reland of 3fc96631

The private method on_event in timeout proc is now renamed to be truly
private.

Original change's description:
> [test] Print hanging tests on linux on test-runner termination
>
> This will print the list of processes still running before and after
> joining workers during termination. This will help debugging hanging
> tests during flake-bisect or with num-fuzzer, which both terminate
> on total timeout and currently still sometimes hang without printing
> processes.
>
> Bug: v8:8292
> Change-Id: I124b65fa35b8d7a6aa198fcf50f2c20df94dc51a
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1735312
> Reviewed-by: Tamer Tas <tmrts@chromium.org>
> Commit-Queue: Michael Achenbach <machenbach@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#63065}

Bug: v8:8292
Change-Id: Ibad1172666d6f4d2c07884a54edfe9d6499b57fe
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1735318Reviewed-by: Tamer Tas <tmrts@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63072}