Extend unit test coverage to more useradm functions and polish (#576)

* Extend unit test coverage to more `useradm` functions and test more thoroughly
for expected files in `create_user` to match checks in new `delete_user` test.

* Update to use `usersupp` addition of `OTHER_USER_DN` and `NO_SUCH_USER_DN` consts.

* Apply some suggested `make link-python` fixes.

* Fixed buggy `home_dir` construction (Thanks @Martin-Rehr).

Author: jonasbardino