Support re-entrant sessions that talk to the same server

In the Wingman test suite, I have 100+ golden tests. Currently, every one of them needs to spin up and subsequently tear down the server, which seems like a lot of wasted effort. I'd like to spin up a server before the first test runs, and have each test communicate with the same server.

I tried to roll my own, but running a `Session` is very opinionated about killing the server when it's done.