Avoid test failures on Darwin 7.9.0 (MacOS X 10.3.9)

Avoid test failures on Darwin 7.9.0 (MacOS X 10.3.9)
* tests/chgrp/basic: Don't let failure by chgrp to set the
group of a symlink make this test fail.  Do give a diagnostic.
In the chgrp-no-change-ctime test, add darwin7.9.0 as another
known-failing system.
When failing on some other system, print $host_triplet, too.
Reported by Peter Fales.