rails--rails/railties/test/application/integration_test_case_test.rb

require 'isolation/abstract_unit'

module ApplicationTests
  class IntegrationTestCaseTest < ActiveSupport::TestCase
    include ActiveSupport::Testing::Isolation

    setup do
      build_app
      boot_rails
    end

    teardown do
      teardown_app
    end

    test "resets Action Mailer test deliveries" do
      script('generate mailer BaseMailer welcome')

      app_file 'test/integration/mailer_integration_test.rb', <<-RUBY
        require 'test_helper'

        class MailerIntegrationTest < ActionDispatch::IntegrationTest
          setup do
            @old_delivery_method = ActionMailer::Base.delivery_method
            ActionMailer::Base.delivery_method = :test
          end

          teardown do
            ActionMailer::Base.delivery_method = @old_delivery_method
          end

          2.times do |i|
            define_method "test_resets_deliveries_\#{i}" do
              BaseMailer.welcome.deliver_now
              assert_equal 1, ActionMailer::Base.deliveries.count
            end
          end
        end
      RUBY

      output = Dir.chdir(app_path) { `bin/rails test 2>&1` }
      assert_equal 0, $?.to_i, output
      assert_match(/0 failures, 0 errors/, output)
    end
  end
end