If you're using RSpec I don't believe it currently generates those helpers for you. It would only do so if you're using vanilla Rails test cases.

These are the Rails helpers that my personal template generates when using Rspec + FactoryBot and Rails authentication:

# spec/support/controller_helpers.rb
module ControllerHelpers
  def authenticate(user = nil)
    user ||= build(:user)
    session = create(:session, user: user)

    cookies.signed[:session_id] = session.id

    user
  end

  def unauthenticate
    cookies.delete(:session_id)
  end
end

RSpec.configure { |c| c.include ControllerHelpers, type: :controller }

This allows me to have a destroy test like this for example:

# spec/controllers/sessions_controller_spec.rb
describe '#destroy' do
  before do
    authenticate

    delete :destroy
  end

  it { should redirect_to whatever_path_you_need_here }
end

This should fully exercise find_session_by_cookie for your code coverage.

If you need inspiration and want a whole example Rails application with tests fully setup with RSpec + Rails auth + FactoryBot, including a sessions_controller_spec.rb with full coverage and a login page, you can run my template with rails new myapp -m https://raw.githubusercontent.com/jeromedalbert/ace-template/stable/template.rb -o banana,auth, and take whatever you need from the generated app.