wgengine/magicsock: add an option to disable legacy peer handling.

Used in tests to ensure we're not relying on behavior we're going
to remove eventually.

Signed-off-by: David Anderson <danderson@tailscale.com>

wgengine/magicsock/magicsock_test.go

@@ -1485,6 +1485,7 @@ func BenchmarkReceiveFrom(b *testing.B) {
 		EndpointsFunc: func(eps []string) {
 			b.Logf("endpoints: %q", eps)
 		},
+		DisableLegacyNetworking: true,
 	})
 	if err != nil {
 		b.Fatal(err)