net/{interfaces,netns}: add some new tests, missed from prior commit

I meant for these to be part of 52e24aa966.

net/interfaces/interfaces_default_route_test.go

@@ -0,0 +1,17 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build linux darwin,!redo
+
+package interfaces
+
+import "testing"
+
+func TestDefaultRouteInterface(t *testing.T) {
+	v, err := DefaultRouteInterface()
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("got %q", v)
+}