Skip the 004_io_direct.pl test if a pre-flight check fails.

Skip the 004_io_direct.pl test if a pre-flight check fails.

The test previously had a list of OSes that direct I/O was expected to
work on.  That worked well enough for the systems in our build farm, but
didn't survive contact with the Debian build bots running on tmpfs via
overlayfs.  tmpfs does not support O_DIRECT, but we don't want to
exclude Linux generally.

The new approach is to try to create an empty file with O_DIRECT from
Perl first.  If that fails, we'll skip the test and report what the
error was.

Reported-by: Christoph Berg <[email protected]>
Reviewed-by: Dagfinn Ilmari Mannsåker <[email protected]>
Reviewed-by: Andrew Dunstan <[email protected]>
Discussion: https://postgr.es/m/ZDYd4A78cT2ULxZZ%40msg.df7cb.de