[dev.fuzz] internal/fuzz: fix context cancellation for coordinator

[dev.fuzz] internal/fuzz: fix context cancellation for coordinator

Previously, performing a SIGINT would cause the coordinator
to write a crash to testdata, and would continue to run
despite being interupted.

Also includes a few small cleanups.

Change-Id: Ia3cf7cd231c30ac9ad2a61f4935aa543e241f60d
Reviewed-on: https://go-review.googlesource.com/c/go/+/283634
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>