main.rb

class NotANumberException < StandardError
end

class FizzBuzz
  
  def initialize(numero)
    raise NotANumberException unless numero.is_a? Integer
    @numero = numero
  end
  
  def calcula()
    valor = @numero
    
    if (@numero % 15 == 0)
      valor = "FizzBuzz"
    elsif (@numero % 3 == 0 ) 
      valor =  "Fizz"
    elsif (@numero % 5 == 0)
      valor = "Buzz" 
    end
    
    valor
  end
end

main_test.rb

require 'test/unit'
require 'main'
require 'rubygems'
require 'redgreen'

class FizzBuzzTest < Test::Unit::TestCase

  def test_fizz_buzz_recebe_um_numero
    fizz_buzz = FizzBuzz.new(1)
  end
  
  def test_fizz_buzz_um_retorna_um
    fizz_buzz = FizzBuzz.new(1)
    assert_equal 1, fizz_buzz.calcula()
  end

  def test_fizz_buzz_tres_retorna_fizz
    fizz_buzz = FizzBuzz.new(3)
    assert_equal "Fizz", fizz_buzz.calcula()
  end

  def test_fizz_buzz_seis_retorna_fizz
    fizz_buzz = FizzBuzz.new(6)
    assert_equal "Fizz", fizz_buzz.calcula()
  end

  def test_fizz_buzz_cinco_retorna_buzz
    fizz_buzz = FizzBuzz.new(5)
    assert_equal "Buzz", fizz_buzz.calcula()
  end
  
  def test_fizz_buzz_quinze_retorna_fizzbuzz
    fizz_buzz = FizzBuzz.new(15)
    assert_equal "FizzBuzz", fizz_buzz.calcula()
  end
  
  def test_fizz_buzz_nao_recebe_string
    #fizz_buz = FizzBuzz.new("teste")
    assert_raises NotANumberException do
       FizzBuzz.new("teste")
    end
  end

  def test_fizz_buzz_nao_recebe_float
    #fizz_buz = FizzBuzz.new("teste")
    assert_raises NotANumberException do
       FizzBuzz.new(2.2)
    end
  end
  
end

watchr.config

watch(/\.rb$/) do |filw|
  system('clear && ruby -Itest main_test.rb')
end