It was relying on a higher-than-default fd limit; cut it down to
more than 256 but rather less than 1024, since the default Linux
limit is 1024 fds per user.
Also automatically skip this test if our rlimit is too small.
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=88998
Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>