selftests/pidfd: third test for multi-threaded exec polling

Ensure that during a multi-threaded exec and premature thread-group
leader exit no exit notification is generated.

Link: https://lore.kernel.org/r/20250320-work-pidfs-thread_group-v4-4-da678ce805bf@kernel.org


Signed-off-by: Christian Brauner <brauner@kernel.org>