Make DWDS test server more re-usable

Currently DWDS' `TestServer` is only being used by `context.dart`. That's because so much set up is required for the parameters passed to `TestServer.start` that we are only doing so from the `TextContext` set up.

We should move a lot of that logic to `TestServer` itself so that it is lightweight to start with default arguments.

I would like to also make it accessible outside of the DWDS directory (that way we use the test server to start an app to connect to with the debug extension, and then move the debug extension tests to their own directory instead of the dwds directory)