selftests/landlock: Skip scoped_signal subtest with MSG_OOB if not available

selftests/landlock: Skip scoped_signal subtest with MSG_OOB if not available

MSG_OOB might be disabled in the kernel for unix sockets (by not
selecting CONFIG_AF_UNIX_OOB), and in this case the related tests
of the scoped_signal_test are currently failing.  Add a runtime
probe using socketpair() to detect MSG_OOB support and skip the
test gracefully if it is unavailable.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Link: https://patch.msgid.link/20260710081642.405916-1-thuth@redhat.com
Cc: stable@vger.kernel.org
Fixes: f34e9ce5f479 ("selftests/landlock: Test signal created by out-of-bound message")
Signed-off-by: Mickaël Salaün <mic@digikod.net>