test: verify fdinfo for pidfd of reaped process

Test that the fdinfo field of a pidfd referring to a dead process
correctly shows Pid: -1 and NSpid: -1.

Cc: Christian Kellner <christian@kellner.me>
Cc: linux-kselftest@vger.kernel.org
Reviewed-by: Christian Kellner <christian@kellner.me>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20191017101832.5985-2-christian.brauner@ubuntu.com