wgengine/monitor: don't call LinkChange when interfaces look unchanged

Basically, don't trust the OS-level link monitor to only tell you
interesting things. Sanity check it.

Also, move the interfaces package into the net directory now that we
have it.

net/interfaces/interfaces_test.go

@@ -0,0 +1,31 @@
+// Copyright (c) 2020 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.
+
+package interfaces
+
+import (
+	"net"
+	"testing"
+)
+
+func TestIsTailscaleIP(t *testing.T) {
+	tests := []struct {
+		ip   string
+		want bool
+	}{
+		{"100.81.251.94", true},
+		{"8.8.8.8", false},
+	}
+	for _, tt := range tests {
+		ip := net.ParseIP(tt.ip)
+		if ip == nil {
+			t.Fatalf("failed to parse IP %q", tt.ip)
+		}
+		got := IsTailscaleIP(ip)
+		if got != tt.want {
+			t.Errorf("F(%q) = %v; want %v", tt.ip, got, tt.want)
+		}
+	}
+
+}