ssh: fix a bug in dropping privileges on Linux

On Linux syscall.Setuid() is not supported in go. So running unit tests
with root privileges fails when calling Setuid.
Let's skip the test on Linux, if euid is 0.

ssh/known_hosts_test.go

@@ -19,6 +19,7 @@ import (
 	"io/ioutil"
 	"net"
 	"os"
+	"runtime"
 	"strconv"
 	"syscall"
 	"testing"
@@ -165,7 +166,13 @@ func TestWrongHostKeyFile(t *testing.T) {
 	// If run as root, drop privileges temporarily
 	if id := syscall.Geteuid(); id == 0 {
 		if err := syscall.Setuid(12345); err != nil {
-			t.Fatalf("error setting uid: %v", err)
+			if runtime.GOOS == "linux" {
+				// On Linux syscall.Setuid is not supported, so we should not fail
+				// the test, but just skip for now. - dpark 20161021
+				t.Skipf("error setting uid: %v", err)
+			} else {
+				t.Fatalf("error setting uid: %v", err)
+			}
 		}
 		defer syscall.Setuid(id)
 	}