selftests/landlock: Add LANDLOCK_RESTRICT_SELF_TSYNC tests

selftests/landlock: Add LANDLOCK_RESTRICT_SELF_TSYNC tests

Exercise various scenarios where Landlock domains are enforced across
all of a processes' threads.

Test coverage for security/landlock is 91.6% of 2130 lines according to
LLVM 21.

Cc: Andrew G. Morgan <morgan@kernel.org>
Cc: John Johansen <john.johansen@canonical.com>
Cc: Paul Moore <paul@paul-moore.com>
Signed-off-by: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20251127115136.3064948-3-gnoack@google.com
[mic: Fix subject, use EXPECT_EQ(close()), make helpers static, add test
coverage]
Signed-off-by: Mickaël Salaün <mic@digikod.net>