test infra: come up with smart ideas to test PUSH_UPDATE

so we do have server/client tests, but they do not come near the PUSH_UPDATE infra - and it does not really fit easily into the "do magic by --auth-user-pass-verify or --client-connect scripts", as it needs to be driven by the management interface.

So we need something like

• client sents an UV_I_WANT_UPDATES message
• "something new" listens on the server side management interface, and on seeing such a client, sends an PUSH_UPDATE message (content to be agreed between "t_client instance" and "server instance")
• the t_client instance actually waits for the PUSH_UPDATE and then goes ahead and pings - so this could be PUSH_UPDATE'ing --dns (ping v4.open.vpn) or --route (not all necessary routes in inital PUSH_REPLY), maybe also --ifconfig (push something nonworking first, then update with proper address)
• server can verify that --learn-address actually works for --ifconfig

these are somewhat crucial code paths on both ends, so having automated tests is needed.