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

(cherry picked from commit 8f75390c66bdcde95e1b383aecaa27b4adf88279)