syscall: accept more variants of id output when testing as root

This should fix the report at #16224, and also fixes running the test as
root on my Ubuntu Trusty system.

Fixes #16224.

Change-Id: I4e3b5527aa63366afb33a7e30efab088d34fb302
Reviewed-on: https://go-review.googlesource.com/24670
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/syscall/exec_linux_test.go

@@ -200,8 +200,10 @@ func TestGroupCleanup(t *testing.T) {
 	}
 	strOut := strings.TrimSpace(string(out))
 	expected := "uid=0(root) gid=0(root) groups=0(root)"
-	if strOut != expected {
-		t.Fatalf("id command output: %s, expected: %s", strOut, expected)
+	// Just check prefix because some distros reportedly output a
+	// context parameter; see https://golang.org/issue/16224.
+	if !strings.HasPrefix(strOut, expected) {
+		t.Errorf("id command output: %q, expected prefix: %q", strOut, expected)
 	}
 }
 
@@ -230,10 +232,17 @@ func TestGroupCleanupUserNamespace(t *testing.T) {
 		t.Fatalf("Cmd failed with err %v, output: %s", err, out)
 	}
 	strOut := strings.TrimSpace(string(out))
-	// there are two possible outs
-	expected1 := "uid=0(root) gid=0(root) groups=0(root)"
-	expected2 := "uid=0(root) gid=0(root) groups=0(root),65534(nobody)"
-	if strOut != expected1 && strOut != expected2 {
-		t.Fatalf("id command output: %s, expected: %s or %s", strOut, expected1, expected2)
+
+	// Strings we've seen in the wild.
+	expected := []string{
+		"uid=0(root) gid=0(root) groups=0(root)",
+		"uid=0(root) gid=0(root) groups=0(root),65534(nobody)",
+		"uid=0(root) gid=0(root) groups=0(root),65534(nogroup)",
+	}
+	for _, e := range expected {
+		if strOut == e {
+			return
+		}
 	}
+	t.Errorf("id command output: %q, expected one of %q", strOut, expected)
 }