[dev.fuzz] testing/internal/testdeps: use signal.NotifyContext
[dev.fuzz] testing/internal/testdeps: use signal.NotifyContext

In RunFuzzWorker and CoordinateFuzzing, use signal.NotifyContext
(new in 1.16) to cancel the context in response to SIGINT. This is
shorter and more correct than what we were doing before.

Change-Id: Id7e9a58e9dd992dffb86e4e0f2e3efd117d03c47
Reviewed-on: https://go-review.googlesource.com/c/go/+/290691
Trust: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
Run-TryBot: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Katie Hockman <e321a8fd89c4908465ddaf8d5d0d62480efacb63@golang.org>

cloudflare/go,golang/go,golang/go,CAFxX/go,golang/go,cloudflare/go,golang/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,cloudflare/go,cloudflare/go,cloudflare/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,nawawi/go