lib: fix logic error in strncpy

Fix error caught by gcc:
tst_tmpdir.c: In function 'tst_rmdir':
tst_tmpdir.c:202:39: warning: argument to 'sizeof' in 'strncpy' call
	is the same expression as the destination; did you mean to provide
	an explicit length? [-Wsizeof-pointer-memaccess]
    strncpy(parent_dir, TESTDIR, sizeof(parent_dir));
                                 ^

Signed-off-by: Mike Frysinger <vapier@gentoo.org>