sticky-bit.t test operates on temporary directory now

On some systems (e.g. FreeBSD, probably all *BSD systems) setting the
sticky bit on plain files is not allowed. Only directories may get the
sticky bit here:

    $ touch file
    $ chmod +t file
    chmod: file: Inappropriate file type or format

Fix sticky-bit.t test to use a temporary directory, not a temporary
file.

Overview of test failures may be seen here: http://217.199.168.174/cgi-bin/cpantestersmatrix.pl?dist=File-chmod

Regards, Slaven