How to check Hubot log output in test script

I'm writing a test for my Hubot (which acts as a Slack bot). Triggered by certain Slack messages, the bot sends an HTTP POST request to a separate Rails app. How can I check in my test script that the HTTP request has been sent? My guess was that I should check the contents of the robot.logger (please let me know if there's a better way) - but if so, how can I access the log in the test?

Hubot script (basically, it informs the Rails app about a user who is leaving the office to take a break):

module.exports = (robot) ->
  robot.respond /off to lunch/i, (res) ->
    res.reply('Later alligator')
    robot.logger.info "This user is going on lunch break: #{res.message.user.id}"

    data = JSON.stringify({
      slack_user_id: res.message.user.id
    })
    robot.http(process.env.RAILS_APP_URL + '/break')
      .header('Content-Type', 'application/json')
      .post(data) (err, resp, body) ->
        if err
          robot.logger.info "Encountered an error. #{err}"
          res.reply('Sorry, there was an error recording your break time')
        else
          robot.logger.info 'Successfully sent HTTP POST request to Rails app'

Log output when I execute this script:

INFO This user is going on lunch break: [SLACK_USER_ID]
INFO Successfully sent HTTP POST request to Rails app

As I mentioned above, I'd like to check in my test script that the HTTP request was sent, by asserting that the log is going to include the message 'Successfully sent HTTP POST request to Rails app'. However, I don't know how to access the Hubot's log in my test. I thought it would have something to do with process.stdout because the bot logs to stdout, but I couldn't get it to work.

Test script:

Helper = require('hubot-test-helper')
helper = new Helper('../scripts/break-start.coffee')
request = require('request')
expect = require('chai').expect
nock = require('nock')

describe 'bot responds to user message and sends ', ->
  beforeEach ->
    # Set up the room before running the test.
    @room = helper.createRoom()

    # Set up a request interceptor.
    nock(process.env.RAILS_APP_URL)
      .post('/break', { slack_user_id: 'bob' })
      .reply(200)

  afterEach ->
    # Tear down the room after the test to free up the listener.
    @room.destroy()

  context 'user sends lunch message', ->
    beforeEach ->
      @room.user.say('bob', '@hubot Off to lunch')

    it 'responds to users who are off to lunch', ->
      expect(@room.messages).to.eql [
        ['bob', '@hubot Off to lunch']
        ['hubot', '@bob Later alligator']
      # I want to do something like this:
      # expect(robot.log).include('Successfully sent HTTP POST request to Rails app')

Of course, I can see in the console log when I run the test that the HTTP request is being sent, but I'd also like to assert it so that the test fails if the request is not sent.

Log output when test is executed:

INFO This user is going on lunch break: bob
✓ responds to users who are off to lunch
INFO Successfully sent HTTP POST request to Rails app

Thank you for any help in advance.