princess.rb

#!/usr/bin/env ruby
gem 'minitest', '>= 5.0.0'
require 'minitest/autorun'
require_relative 'princess'
require 'pry'

class ArrayTest < Minitest::Test

  def test_princess_play
    assert_output('You are knocked from the round.') { Princess.play }
  end

end

class Princess

  def self.play
    puts "You are knocked from the round."
  end

end