cat.rb

class Cat

  def initialize
  end

  def to_s
    return "This is a string"
  end
end

cat_test.rb

require 'minitest/autorun'
require 'minitest/pride'
require_relative 'cat'

class CatTest < Minitest::Test

  def test_to_s
    cat = Cat.new
    result = cat.to_s

    assert_equal "This is a string", result
  end

end