Skip ext/GDBM_File/t/fatal.t in Darwin, too flaky.

(random crashes at destruction time)

See https://rt.perl.org/Public/Bug/Display.html?id=117967
and jhi ran into this in OS 10.9.4 + blead + gdbm 1.11
(gdbm via macports, but shouldn't matter much: flaky is flaky).