unix: use vm.swap_total for FreeBSD TestSysctlUint64

Fixes golang/go#21375

Change-Id: Ibb33df9d71eb3a935a1f8f9f0ad1f180211e8ce4
Reviewed-on: https://go-review.googlesource.com/54390
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/syscall_freebsd_test.go

@@ -21,7 +21,7 @@ import (
 )
 
 func TestSysctlUint64(t *testing.T) {
-	_, err := unix.SysctlUint64("security.mac.labeled")
+	_, err := unix.SysctlUint64("vm.swap_total")
 	if err != nil {
 		t.Fatal(err)
 	}