Use higher wal_level for 004_io_direct.pl.

Use higher wal_level for 004_io_direct.pl.

The new direct I/O test deliberately uses a very small shared_buffers to
force some disk transfers without making the data set large and slow,
but ran into a problem with wal_level = minimal: log_newpage_range()
pins many buffers, leading to a few intermittent "no unpinned buffers
available" errors.

We could presumably fix that by adjusting shared_buffers, but crake
seems to be trying to tell us something interesting with these settings,
so let's just avoid wal_level = minimal in this test for now.

Reported-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20230408060408.n7xdwk3mxj5oykt6%40awork3.anarazel.de