Integrate contrib/file/tests with kyua/atf

This could be done better by making each test a separate ATF test case.
This exercise is left for the reader.

Reviewed by: delphij (earlier version)
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D34303

(cherry picked from commit 1581ec9a4502c1f0e4f0ce290190864abf946d6f)