Minitest spec rails parallel testing fails

shell

gem install rails -v 6.0.0.beta2
rails _6.0.0.beta2_ new parallel-testing
cd parallel-testing
bin/rails g scaffold User username:string
echo "gem 'minitest-spec-rails'" >> Gemfile
bundle
bin/rails db:migrate
# Replace testcase with the one provided in this gist
bin/rails test

users_controller_test.rb

require 'test_helper'

class UsersControllerTest < ActionDispatch::IntegrationTest
  setup do
    @user = users(:one)
  end

  # Disabling 'parallelize' in test_helper ensures these tests pass
  describe 'POST create' do
    it 'should create user' do
      assert_difference('User.count') do
        post users_url, params: { user: { username: @user.username } }
      end
      assert_response :redirect
    end

    it 'should create user with different cookie' do
      assert_difference('User.count') do
        post users_url, params: { user: { username: @user.username } }
      end
      assert_response :redirect
    end
  end

  # These tests always pass
  test "should create user" do
    assert_difference('User.count') do
      post users_url, params: { user: { username: @user.username } }
    end

    assert_redirected_to user_url(User.last)
  end

  test "should create another user" do
    assert_difference('User.count') do
      post users_url, params: { user: { username: @user.username } }
    end

    assert_redirected_to user_url(User.last)
  end
end