gem install rspec
rspec --init

foo.rb

class Foo
  def sum(a, b)
    a + b
  end
end

spec/foo_spec.rb

require 'rspec'
require './foo.rb'

describe Foo do
  it 'Summa 2 and 3 do 5' do
    dog = Foo.new
    expect(dog.sum(2, 3)).to eq(5)
  end
end

bash:

rspec