Restore pathing for factor(1), which unnecessarily diverged in r278616

factor lives in /usr/games/, not /usr/bin, in NetBSD.

The correct way to handle this is do on-the-fly manipulation of the test
script via ATF_TESTS_SH_SED_<foo>, not by modify the pathing directly in
the test script.

This is being done to resolve an unnecessary conflict made when pulling
back ^/head@r309469 (contrib/netbsd-tests update) to ^/stable/10.

No functional change

MFC after: now