tests/qtest: pass stdout/stderr down to subtests

tests/qtest: pass stdout/stderr down to subtests

When trying to work out what the virtio-net-tests where doing it was
hard because the g_test_trap_subprocess redirects all output to
/dev/null. Lift this restriction by using the appropriate flags so you
can see something similar to what the vhost-user-blk tests show when
running.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20220407150042.2338562-1-alex.bennee@linaro.org>

Message-Id: <20220802095010.3330793-15-alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>