Add DeleteUser method to ControlServer interface and implement it in
HeadscaleInContainer to enable testing user deletion scenarios.
Add two integration tests for issue #2967:
- TestACLGroupWithUnknownUser: tests that valid users can communicate
when a group references a non-existent user
- TestACLGroupAfterUserDeletion: tests connectivity after deleting a
user that was referenced in an ACL group
These tests currently pass but don't fully reproduce the reported issue
where deleted users break connectivity for the entire group.
Updates #2967
98c0817b95