get agentConfig() {
    return {
      host: this.host,
      port: this.port,
      license_key: 'testing',
      certificates: [this.cert]
    }
  }
A configuration object that can be passed to an "agent" instance so that
the agent will communicate with this test server instead of the real
server.

Important: the `.listen` method must be invoked first in order to have
the `host` and `port` defined.

@returns {object}