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.