14o. FloripaOnRails - I want to test my code but how to do it

bdd.feature

Feature: champion uses dragonfly to process images
  In order to be a champion
  A user uses dragonfly

  Background:
    Given we are using the app for images
    Given a stored image "beach.png" with dimensions 200x100

  Scenario: Go to url for original
    When I go to the url for "beach.png", with format 'png'
    Then the response should be OK
    And the response should have mime-type 'image/png'
    And the image should have width '200'
    And the image should have height '100'
    And the image should have format 'png'

  Scenario: Go to url for changed format version
    When I go to the url for "beach.png", with format 'gif'
    Then the response should be OK
    And the response should have mime-type 'image/gif'
    And the image should have width '200'
    And the image should have height '100'
    And the image should have format 'gif'

minitest_example.rb

class Calculator
  def self.sum(a, b)
    a + b
  end
end

require 'minitest/autorun'

class CalculatorTest < Minitest::Unit::TestCase
  def test_sum_with_positive_numbers
    assert_equal(3, Calculator.sum(1, 2))
  end

  def test_sum_with_negative_numbers
    assert_equal(1, Calculator.sum(-1, 2))
  end
end