test: check generated files for ansible_managed, fingerprint

Add the following files: tests/tasks/check_header.yml and
tests/templates/get_ansible_managed.j2.
Use check_header.yml to check generated files for the ansible_managed
and fingerprint headers.
check_header.yml takes two parameters:

* `__file` - required - the full path of the file to check e.g. `/etc/realmd.conf`
* `__fingerprint` - required - the fingerprint string `system_role:$ROLENAME` e.g.
  `__fingerprint: "system_role:postfix"`
* `__comment_type` - optional, default `plain` - the type of comments used

e.g. `__comment_type: c` for C/C++-style comments.  `plain` uses `#`.
See https://docs.ansible.com/ansible/latest/playbook_guide/playbooks_filters.html#adding-comments-to-files
for the different types of comment styles supported.

Example:
```
- name: Check realmd header for ansible_managed, fingerprint
  include_tasks: tasks/check_header.yml
  vars:
    __file: /etc/realmd.conf
    __fingerprint: "system_role:ad_integration"
```

tests/tasks/check_header.yml

@@ -0,0 +1,15 @@
+# SPDX-License-Identifier: MIT
+---
+- name: Get file
+  slurp:
+    path: "{{ __file }}"
+  register: __content
+
+- name: Check for presence of ansible managed header, fingerprint
+  assert:
+    that:
+      - ansible_managed in content
+      - __fingerprint in content
+  vars:
+    content: "{{ __content.content | b64decode }}"
+    ansible_managed: "{{ lookup('template', 'get_ansible_managed.j2') }}"

tests/templates/get_ansible_managed.j2

@@ -0,0 +1 @@
+{{ ansible_managed | comment(__comment_type | d("plain")) }}

tests/tests_basic_join.yml

@@ -29,3 +29,9 @@
       register: __stat_result
       failed_when: not __stat_result.stat.exists
       changed_when: false
+
+    - name: Test - Check realmd headers
+      include_tasks: tasks/check_header.yml
+      vars:
+        __file: /etc/realmd.conf
+        __fingerprint: "system_role:ad_integration"