[Feature Request] `TestWorkflowEnvironment` should support non-spawned servers

[Sushisource]

Right now there's not an easy way to run integration tests against an already-extant server. The tests always spawn one for you. It's nice to be able to use UI, maintain some history of runs, etc, while working through something.

An env var where you specify the addr/port of an already-on server would be great.

[bergundy]

In Python you can start a test env from a client.

We should support instantiation from a Connection. I would rather also have the worker be powered by the same connection to avoid translating between the objects, the pure JS Connection class has some options that can't be translated to NativeConnection.