What to do when funded channel node disappears I would like to test this scenario on a testnet.

I funded a channel with one testnet node.

Assume that node suddenly vanishes and doesn't come back up for, say, another 4 weeks, or never.

What is supposed to happen? How am I supposed to detect and react to this?
Generally your lightning node will be able to tell which peers are online (in clightning with listpeers command)

If the other node is offline you can force close the channel by publishing your channel state.. Since the other node has signed your commitment tx as part of the protocol there is no need for it to be online or to help closing the channel.