Add a comment explaining (probably) why this

test fails on OpenBSD 3.2 when run in an NFS-mounted directory.

tests/chgrp/basic

@@ -81,6 +81,11 @@ test "$VERBOSE" = yes && set +x
   chgrp $g2 g
   sleep 1
   chgrp $g1 f
+
+  # The following no-change chgrp command is supposed to update f's ctime,
+  # but on OpenBSD, it appears to be a no-op for some file system types
+  # (at least NFS) so g's ctime is more recent.  This is not a big deal;
+  # this test works fine when the files are on a local file system (/tmp).
   chgrp '' f
   ls -c -t f g