selftests/landlock: Add supports_filesystem() helper

Replace supports_overlayfs() with supports_filesystem() to be able to
check several filesystems.  This will be useful in a following commit.

Only check for overlay filesystem once in the setup step, and then rely
on self->skip_test.

Cc: Guenter Roeck <groeck@chromium.org>
Cc: Jeff Xu <jeffxu@google.com>
Link: https://lore.kernel.org/r/20230612191430.339153-4-mic@digikod.net


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