This reverts commit 3fc96631.

Reason for revert: _on_event clashes with a the same function in timeout proc.

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}

TBR=machenbach@chromium.org,tmrts@chromium.org

Change-Id: Ic29013e92c77a9905e06ee13a2449e033285eb36
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:8292
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1735317Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63066}