syscall: fix TestCloneNEWUSERAndRemapRootEnableSetgroups on linux

Despite its name, it did the same thing as
TestCloneNEWUSERAndRemapRootDisableSetgroups in old code. So fix it.

Change-Id: I21265e539179c51980e16f07a553a9bf5bb7903f
Reviewed-on: https://go-review.googlesource.com/35273
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/syscall/exec_linux_test.go

@@ -133,7 +133,7 @@ func TestCloneNEWUSERAndRemapRootEnableSetgroups(t *testing.T) {
 	if os.Getuid() != 0 {
 		t.Skip("skipping root only test")
 	}
-	testNEWUSERRemap(t, 0, 0, false)
+	testNEWUSERRemap(t, 0, 0, true)
 }
 
 func TestCloneNEWUSERAndRemapNoRootDisableSetgroups(t *testing.T) {