ParseSignal: drop add'l check for numeric signals

The kernel will return an error anyway.

Drop the test case as well.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

signals_unix.go

@@ -33,11 +33,7 @@ import (
 func ParseSignal(rawSignal string) (syscall.Signal, error) {
 	s, err := strconv.Atoi(rawSignal)
 	if err == nil {
-		signal := syscall.Signal(s)
-		if unix.SignalName(signal) != "" {
-			return signal, nil
-		}
-		return -1, fmt.Errorf("unknown signal %q", rawSignal)
+		return syscall.Signal(s), nil
 	}
 	signal := unix.SignalNum(strings.ToUpper(rawSignal))
 	if signal == 0 {