selftests: add ESRCH tests for pidfd_getfd()

Ensure that pidfd_getfd() reports -ESRCH if the task is already exiting.

Signed-off-by: Tycho Andersen <tandersen@netflix.com>
Link: https://lore.kernel.org/r/20240206192357.81942-1-tycho@tycho.pizza


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