selftests/landlock: Test open() and ftruncate() in multiple scenarios

This test uses multiple fixture variants to exercise a broader set of
scnenarios.

Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20221018182216.301684-7-gnoack3000@gmail.com


Signed-off-by: Mickaël Salaün <mic@digikod.net>