Fix security issue in local session

This issue only applies to unit tests: When test code needs to open a session with administrator, username must be added explicitely in session context when opening it. For instance: // opening a system session Framework.login(); RepositoryManager manager = Framework.getService(RepositoryManager.class); Map<String, Serializable> context = new HashMap<String, Serializable>(); context.put("username", "system"); coreSession = manager.getRepository(repo).open(context);