Integrate contrib/file/tests with kyua/atf

[vangyzen]

Summary

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

MFC after: 1 week
Sponsored by: Dell EMC Isilon

Test Plan

$ pwd
/usr/tests/usr.bin/file

$ kyua test
file_test:contrib_file_tests  ->  passed  [0.281s]

Results file id is usr_tests_usr.bin_file.20220216-155405-724728
Results saved to /home/vangyzen/.kyua/store/results.usr_tests_usr.bin_file.20220216-155405-724728.db

1/1 passed (0 failed)

$ ls | wc -l
     103

$ du -shA
181K	.

[vangyzen]

• fixup: attach tests to the parent Makefile