Wait until server running option

[WVAviator]

Currently, for scripts, wait_until can be either seconds or finished. For servers, neither of these are ideal because they can take a variable amount of time to spin up and will never finish.

A third option, potentially called port_open or otherwise, could be specified. Internally, this would poll the port in question until it has been opened by the server.

setup:
  before_all:
    - description: "start the app server"
      script: "npm start"
      wait_until: port_open 3000

This will require special deserialization logic for serde since it won't be able to differentiate between this string and the 'finished' string as untagged enum variants.