selftests/seccomp: check that a zombie leader doesn't affect others

Ensure that a dead thread leader doesn't prevent installing new filters
with SECCOMP_FILTER_FLAG_TSYNC from other threads.

Signed-off-by: Andrei Vagin <avagin@google.com>
Link: https://lore.kernel.org/r/20240628021014.231976-5-avagin@google.com


Reviewed-by: Tycho Andersen <tandersen@netflix.com>
Signed-off-by: Kees Cook <kees@kernel.org>