test: allow running with fatal warnings given in PERL5OPT.

test: allow running with fatal warnings given in PERL5OPT.

* HACKING: mention running the test suite with
PERL5OPT=-Mwarnings=FATAL,all  in the environment at new Perl (and
Automake releases), to try to keep up with new Perl warnings.
Suggestion from Jacob Bachmeyer,
https://lists.gnu.org/archive/html/automake/2025-02/msg00001.html
* t/parallel-tests-log-compiler-example.sh: explicitly use warnings
so that this PERL5OPT setting won't be a fatal error.
* NEWS: formatting.