Refactor tests to use abstration around server and client setup for integration tests

[jfhbrook]

Via @mk-pmb :

I think a good solution would be to refactor a lot of these patterns into a test helper module (maybe setup-loopback-listen):

• Find a random port (retry on EADDRINUSE), or get an OS-assigned address (demo below)
• If given just options instead of an ecstatic instance, create one with these options. (Might be abstracted to support other (req, res) handlers or middleware.)
• Configure events and listen. When listening:
  • Lookup port if set by OS
  • Provide a pre-configured instance of request, and the base URL where the ecstatic instance is listening. (Also refs to the server and the ecstatic instance. The latter because a subsequent test shouldn't have to know whether if was supplied or created with options.)

See: #175