Rails and MiniTest: add additional folder

Rails and MiniTest: add additional folder

I use Ruby 2 and Rails 4. I have a folder test/lib, where a few tests are
located. But running rake test does not use them. Only the other tests
(models, controllers, ...) are running.
Where do I have to add the lib folder?
I already tried MiniTest::Rails::Testing.default_tasks << 'lib', but I get
NameError Exception: uninitialized constant MiniTest::Rails. I did not add
the minitest gem to my Gemfile, because Ruby 2 uses it by default.