Create a test mountable rails engine and mount through a parent rails routes

host_application_routes.rb

mount RailsEngineTest::Engine => 'api/v3/'

rails_mountable_engine.rb

module RailsEngineTest
  class Engine < ::Rails::Engine
    isolate_namespace RailsEngineTest
  end

  class FakeLocationsController < ActionController::Base
    protect_from_forgery

    def create
      render text: 'hello create'
    end

    def index
      render text: 'hello index'
    end
  end

  Engine.routes.draw do
    resources :fake_locations, only: %i[create index]
  end
end