selftests/landlock: Add scope and ptrace tracepoint tests

Add trace tests for the landlock_deny_ptrace,
landlock_deny_scope_signal, and landlock_deny_scope_abstract_unix_socket
tracepoints, each placed alongside the functional tests for its
subsystem, mirroring the audit test layout.

Each tracepoint is exercised by a fixture with three variants that pin
both branches of the other-party domain field: denied against an
unsandboxed other party (other-party domain ID 0), denied against a
sandboxed other party (non-zero ID), and an allowed baseline that
records no event.  A second fixture per type exercises an alternate LSM
hook that reaches the same tracepoint with the same other-party domain
ID, since each denial type can be reached through more than one hook.

The datagram abstract-unix variant does not assert peer_pid, which is 0
for a datagram peer (no SO_PEERCRED); sun_path is the reliable peer
identifier.  The ptrace fixtures install a plain domain-creating ruleset
rather than a dedicated flag, since ptrace denial relies on domain
ancestry, not on a specific scoped flag.  The fixtures unshare the mount
namespace and remount / as MS_PRIVATE before mounting tracefs so the
helper instance is visible only to the test process.

Cc: Günther Noack <gnoack@google.com>
Cc: Tingmao Wang <m@maowtm.org>
Link: https://patch.msgid.link/20260811094338.288094-18-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>

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