Refactor test from `pysymoro/tests/test.py`
Add unittest for `symoroutils/filemgr.py`

Add unittest for `symoroutils/filemgr.py` in
`symoroutils/tests/test_filemgr.py`.
Remove some of the doctest code from `symoroutils/filemgr.py` and move
to the corresponding unittest file.