syscall: add missing err check in test

Follow CL 75810 which did the same for x/sys/unix.

Change-Id: I6314a97989631d589369c7b6001d1523ec2cc242
Reviewed-on: https://go-review.googlesource.com/80555
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/syscall/syscall_unix_test.go

@@ -180,6 +180,9 @@ func TestPassFD(t *testing.T) {
 		uc.Close()
 	})
 	_, oobn, _, _, err := uc.ReadMsgUnix(buf, oob)
+	if err != nil {
+		t.Fatalf("ReadMsgUnix: %v", err)
+	}
 	closeUnix.Stop()
 
 	scms, err := syscall.ParseSocketControlMessage(oob[:oobn])