Remove all `include ActiveSupport::Testing::MethodCallAssertions`
in actioncable's tests since we can do it only in `ActionCable::TestCase`
in order to prevent code duplication.
We use the same approach for other modules of Rails.
We have defined `ActionCable::TestCase` in `actioncable/test/test_helper.rb`
that we can use in order to prevent code duplication and build common
interface for actioncable's test.
