tsnet: add test for packet filter generation from netmap

This is an integration test that covers all the code in Direct, Auto, and
LocalBackend that processes NetMaps and creates a Filter. The test uses
tsnet as a convenient proxy for setting up all the client pieces correctly,
but is not actually a test specific to tsnet.

Updates tailscale/corp#20514

Signed-off-by: James Sanderson <jsanderson@tailscale.com>

ipn/ipnlocal/local_test.go

@@ -5328,7 +5328,7 @@ func TestSrcCapPacketFilter(t *testing.T) {
 		}},
 	})
 
-	f := lb.currentNode().GetFilterForTest()
+	f := lb.GetFilterForTest()
 	res := f.Check(netip.MustParseAddr("2.2.2.2"), netip.MustParseAddr("1.1.1.1"), 22, ipproto.TCP)
 	if res != filter.Accept {
 		t.Errorf("Check(2.2.2.2, ...) = %s, want %s", res, filter.Accept)