RSpec for factorial function

factorial_spec.rb

require_relative '../factorial.rb'

describe "Test the factorial function" do
  it 'returns the right answer for 5' do
    expect(factorial(5)).to eq(120)
  end
  it 'returns the right answer for 10' do
    expect(factorial(10)).to eq(3628800)
  end
  it 'returns an error message for -5' do
    expect(factorial(-5)).to eq(nil)
  end
end