test_sbin_added_to_path from test_tools.py can fail when '/sbin' has subdirectories and one gets returned by os.listdir() as first element
The test_sbin_added_to_path test would then fail with an unhelpful message (diffoscope.exc.RequiredToolNotFound: name_of_the_subdir). This change fixes that situation.
Not sure about how clean the python is, I don't ever use it really.