test: verify the role with molecule

The role was not covered by any automated test, so regressions in the drop-in file handling only surfaced on real
hosts. The scenario starts one container per supported distribution family and covers all four conditional branches of
tasks/main.yaml: a user, a user acting as another user, a group and a group acting as another user.

Beside the created rules the verification asserts that a rule declared as absent is removed again, that the drop-in
directory is included exactly once and that visudo accepts the resulting configuration, because a rejected drop-in file
invalidates every rule of the directory.

Co-authored-by: Copilot <copilot@github.com>

molecule/default/create.yml

@@ -0,0 +1,25 @@
+---
+
+- name: Create
+  hosts: localhost
+  gather_facts: false
+  tasks:
+  - name: Start a container per platform
+    community.docker.docker_container:
+      name: "{{ item.name }}"
+      image: "{{ item.image }}"
+      command: "sleep infinity"
+      state: started
+    loop: "{{ molecule_yml.platforms }}"
+    loop_control:
+      label: "{{ item.name }}"
+
+  - name: Write the instance config
+    ansible.builtin.copy:
+      content: |
+        {% for platform in molecule_yml.platforms %}
+        - instance: {{ platform.name }}
+          connection: community.docker.docker
+        {% endfor %}
+      dest: "{{ molecule_instance_config }}"
+      mode: "0600"