Move testing Object ID to utils

`AN_OBJECT_ID` and `AN_OBJECT_ID_STR` are used across a few of the test modules. Placing them in `utils` allows for easier reuse if they are ever needed in new places or changed.
Add docstrings

`_set_write_concern_as_safe()` and `_set_write_concern_as_w()` are marked as internal methods, so I originally gave them no docstring. Now they at least give some hint of what they do.