xgettext: Perl: Add test against unbounded nesting_depth growth.

xgettext: Perl: Add test against unbounded nesting_depth growth.

Reported by Gavin D. Smith <gavinsmith0123@gmail.com> at
<https://lists.gnu.org/archive/html/bug-gettext/2023-07/msg00008.html>
and <https://savannah.gnu.org/bugs/?64552>.

* gettext-tools/tests/xgettext-perl-stackovfl-5: New file.
* gettext-tools/tests/testdata/xg-pl-so-5.pl: New file, taken from
https://git.savannah.gnu.org/gitweb/?p=texinfo.git;a=blob;f=tp/Texinfo/Convert/HTML.pm;hb=c8d9edd94d9b1a3e675e811208d9e66eaf9a7daa
* gettext-tools/tests/Makefile.am (TESTS): Add xgettext-perl-stackovfl-5.
(EXTRA_DIST): Add testdata/xg-pl-so-5.pl.