Add trace tests for the landlock_enforce_domain event in trace_test.c,
asserting field counts after the syscall returns rather than line
ordering across per-CPU buffers.  They cover single-threaded and TSYNC
enforcement (complete and process_wide set), a multi-threaded non-TSYNC
process (process_wide clear), the single-threaded non-leader edge case,
the flags-only path that creates no domain, and a thread-sync abort that
emits create_domain and free_domain but no enforce_domain.

landlock_enforce_domain is added to the fixture enable path and every
disable list so its zero-events assertions cannot be tripped by a stray
enforcement event.

Test coverage for security/landlock is 91.6% of 2571 lines according to
LLVM 22.

Cc: Günther Noack <gnoack@google.com>
Cc: Tingmao Wang <m@maowtm.org>
Link: https://patch.msgid.link/20260811094338.288094-19-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>

Add tracefs test infrastructure in trace.h: helpers for mounting
tracefs, enabling/disabling events, reading the trace buffer, counting
regex matches, and extracting field values, plus per-event regex
patterns.  The patterns are anchored with ^ and $, verify every
TP_printk field, and use no unescaped dot characters; TRACE_PREFIX
matches the ftrace line format with either the expected task name
(truncated to TASK_COMM_LEN - 1) or "<...>" for an evicted comm cache
entry.

Add trace_test.c with the trace fixture (setup enables all available
events with a PID filter, teardown disables and clears) and the
lifecycle, API, denial-field, and log-flag tests.  Extend the existing
true helper to open its working directory before exiting, triggering a
read_dir denial inside a sandbox, so the exec-based tests can verify
same_exec and the logged decision across an exec.  Move regex_escape()
from audit.h to common.h for shared use by the audit and trace tests.

Enable CONFIG_ENABLE_DEFAULT_TRACERS alongside CONFIG_FTRACE in the
selftest config: CONFIG_FTRACE alone only enables the tracer menu
without activating any tracer, while CONFIG_ENABLE_DEFAULT_TRACERS
selects TRACING (and thus TRACEPOINTS and event tracing) without
depending on architecture-specific syscall tracepoints.  When
CONFIG_FTRACE is disabled it cannot be set, so TRACEPOINTS is correctly
disabled too.

Cc: Günther Noack <gnoack@google.com>
Cc: Tingmao Wang <m@maowtm.org>
Link: https://patch.msgid.link/20260811094338.288094-15-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>