Don't let ATF call basename() and dirname() in a non-standard way.

POSIX basename() and dirname() are allowed to overwrite the input
buffer. The advantage of implementing it that way is that the functions
can be thread-safe, as they don't store the result in a global buffer.

It looks like ATF wants to call basename() and dirname() in the
non-standard way where the argument is of type "const char *". This
change disables the calls to basename() and dirname(), which only seems
to affect the formatting of some rare debug/error messages.

Reviewed by: ngie
Differential Revision: https://reviews.freebsd.org/D6627