users: Functional tests fail when re-run

I have noticed that sometimes, after successfully running the users' functional tests, re-running results in failures. This appears to be because some tests delete users. When users are deleted and recreated with the same name, the old home directory which is not removed carries forward to the newly created user. SSH daemon then refuses to allow the user to login due to mixed up permissions on the ~/.ssh directory. This may be fixed by finding a way to remove the home directory of the user or by creating a username with randomized, non-clashing name.