FR: Document how to write integration and end to end tests when developping service directly connected to tailscale

### What are you trying to do?

I have developed a few utility that directly serve traffic or get traffic through a tailscale connection in Go. I am trying to figure out what is the best way to automate testing for those services. Ideally I would like to be able to have end to end tests with the ability to connect things to a test only network.

### How should we solve this?

Maybe everything is already there and just documentation is needed.

### What is the impact of not solving this?

I can define an interface that match Go Tailscale API and use mockery to generate a mock which allow for some unit testing. This is ok, but not great when you want to ensure things continue to work when you are updating tailscale itself. End to end tests will enable faster update as we can then verify that services are not broken by the update and we can just automatically update pretty much.

### Anything else?

_No response_