embertest.js

test("customers can use a forgot password link", function() {
  ajax('/setup_test_env/signup/preexisting-users').then(function() {
    visit(...);
    fillIn(...);
    click(...);

    andThen(function() {
      equal(find('.errors p').text(), "E-mail taken");
    });
  });
});

states_spec.rb

require 'spec_helper'

describe "states" do
  before do
    # create any global state shared with any states defined below
  end

  context "signup" do
    context "preexisting-users" do
      before do
        FactoryGirl.create(:customer)
      end

      it "sets up expected environment" do
        # This is only used on the rails side to validate that
        # the expected database/server state has been established
        # so that when the JS suite requests this state to be set up,
        # everything will be set up as expected.
        # But actually providing an `it` rails test for 
        # each nested describe/context/context etc environment/state
        # is optional
        expect(Customer.count).to eq(1)
      end
    end
  end
end