tests: use perl, not find+rm, to remove temporary directories

tests: use perl, not find+rm, to remove temporary directories

The File::Path::rmtree function from perl, if used right, is
more reliable and more portable of our past idiom:

    find $dirs -type d ! -perm -700 -exec chmod u+rwx {} ';';
    rm -rf $$dirs || exit 1

at least of the face of unreadable dirs/files and other similar
permission issues (and we have those in our test directories).

In fact, this change fixes some spurious failures seen in
"make distcheck" on Solaris 10.

* t/ax/deltree.pl: New.
* Makefile.am (EXTRA_DIST): Add it.
(clean-local-check): Use it.
* t/ax/test-lib.sh (rm_rf_): Use it.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>