[dev.fuzz] internal/fuzz: handle SIGINT races gracefully

[dev.fuzz] internal/fuzz: handle SIGINT races gracefully

A worker process may be terminated by SIGINT if it doesn't install the
signal handler before SIGINT is delivered. That's likely when TestMain
or the fuzz target setup take a long time. The coordinator now ignores
these errors.

Also, when testdeps.TestDeps.CoordinateFuzzing and RunFuzzWorker
return, they will send a value on the chan passed to signal.Notify
instead of closing it. This should have been obvious in hindsight, but
the signal handler could still send a value on that channel after
those functions return but before the process exits.

Change-Id: Iea2589115f1f9bb7415bb5e7911defee423e642e
Reviewed-on: https://go-review.googlesource.com/c/go/+/284292
Trust: Jay Conrod <jayconrod@google.com>
Reviewed-by: Katie Hockman <katie@golang.org>