..
<%= form_for(@user, url: signup_path) do |f| %>
..

..
get '/signup',  to: 'users#new'
post '/signup',  to: 'users#create'
resources :users
..

..
def new
    @user = User.new
end
def create
    @user = User.new(user_params)
    if @user.save
      redirect_to @user
    else
      render 'new'
    end
end
..

..
test "should keep signup page after failed post request" do
    post signup_path, params:{ user: {  name:  "", email: "user@invalid", password: "123",
                                          password_confirmation: "321" }
                             }
    assert_equal request.fullpath, signup_path
end
..

..
assert_equal request.fullpath, 'signup_path'
..

Failure:
UsersControllerTest#test_should_get_signup_page_after_post_request [/home/ubuntu/workspace/sample_app/test/controllers/users_controller_test.rb:13]:
Expected: "/signup"
  Actual: "signup_path"