cmd/derper,tstest/nettest: skip network-needing test in airplane mode

Not buying wifi on a short flight is a good way to find tests
that require network. Whoops.

Updates #cleanup

Change-Id: Ibe678e9c755d27269ad7206413ffe9971f07d298
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/nettest/nettest.go

@@ -0,0 +1,21 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package nettest contains additional test helpers related to network state
+// that can't go into tstest for circular dependency reasons.
+package nettest
+
+import (
+	"testing"
+
+	"tailscale.com/net/netmon"
+)
+
+// SkipIfNoNetwork skips the test if it looks like there's no network
+// access.
+func SkipIfNoNetwork(t testing.TB) {
+	nm := netmon.NewStatic()
+	if !nm.InterfaceState().AnyInterfaceUp() {
+		t.Skip("skipping; test requires network but no interface is up")
+	}
+}