wget darwin: disable Test-iri-disabled.px

This test fails on MacOS X. The failure has something to do with
the tests expecting filesystems to accept/store filenames as
bytes. On the HFS+ filesystem, however, filenames are Unicode
characters (Normalisation Form D).

This wget ticket appears to be relevant
    http://savannah.gnu.org/bugs/index.php?27541

The maintainer does not seem to think that this test
failure represents a problem in practice:
    http://article.gmane.org/gmane.comp.web.wget.general/8988

But this patch should be revisited/removed if this aspect of wget is
ever addressed in the future.

NB: I'm also a bit concerned/confused that none of the tests mentioned
in that message seem to fail, so I'm not 100% sure if this is the
relevant issue.