tsnet,wgenegine/netstack: add test and fix resource leaks

We were not closing the http.Server and were also not waiting for
netstack to fully close.

Signed-off-by: Maisem Ali <maisem@tailscale.com>

wgengine/netstack/netstack.go

@@ -222,6 +222,7 @@ func Create(logf logger.Logf, tundev *tstun.Wrapper, e wgengine.Engine, mc *magi
 func (ns *Impl) Close() error {
 	ns.ctxCancel()
 	ns.ipstack.Close()
+	ns.ipstack.Wait()
 	return nil
 }