[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>

src/internal/fuzz/sys_unimplemented.go

@@ -29,3 +29,7 @@ func setWorkerComm(cmd *exec.Cmd, comm workerComm) {
 func getWorkerComm() (comm workerComm, err error) {
 	panic("not implemented")
 }
+
+func isInterruptError(err error) bool {
+	panic("not implemented")
+}