ansible/test/integration/targets/handlers/test_notify_included.yml

- name: This worked unintentionally, make sure it does not anymore
  hosts: localhost
  gather_facts: false
  tasks:
    - command: echo
      notify:
        - include_handler

    - meta: flush_handlers

    - command: echo
      notify:
        - handler_from_include
  handlers:
    - name: include_handler
      include_tasks: test_notify_included-handlers.yml