Testing for destroyed users in devise

I am currently having trouble testing for destroying users generated with devise.

require 'test_helper'
class UserTest < ActiveSupport::TestCase
  # test "the truth" do
  #   assert true
  # end
  def setup
    @user = User.new(email: "user@example.com")
  end

  test "Should delete User"do
    @user.save
    assert_difference 'User.count', -1 do
     @user.destroy
    end
  end

end

Currently the tests fail and show that the difference was 0. I was wondering how I could test this (even though I know user.destroy works when I run it in a rails console).

All the best D