file: Support testing multiple magic files.

This is based on upstream b74150490be904801378b2712fe0d532e1700124
but adapted to ATF.

Reviewed by: jlduran@gmail.com, markj, vangyzen
Differential Revision: https://reviews.freebsd.org/D42129

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