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/collections.yml

@@ -0,0 +1,5 @@
+---
+
+collections:
+- name: community.docker
+- name: community.general