Unit tests for parse_common_args() file options

Some tests were already there, but extended the set to consider the
paths that check for file existence on the file system.
The tests should now cover most happy paths within parse_file_args().
Many unhappy paths behave quite unintuitively, so they are not tested
to avoid enforcing such unintuitive behavior.

Change-Id: I21c2dc40a3861784d55b6419dd148d6dda26dde5