quot: Add tests

To facilitate the task, we change the comparison function so that users
with equal filesystem usage are sorted by UID, and add an undocumented
option that prevents quot from replacing numerical UIDs with names. We
also switch from getfsfile(3) to getmntpoint(3) so the first line is
identical regardless of whether we pass quot a mountpoint or a device.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D53133