selftests: add pid namespace ENOMEM regression test

We recently regressed (cf. [1] and its corresponding fix in [2]) returning
ENOMEM when trying to create a process in a pid namespace whose init
process/child subreaper has already died. This has caused confusion at
least once before that (cf. [3]). Let's add a simple regression test to
catch this in the future.

[1]: 49cb2fc42c ("fork: extend clone3() to support setting a PID")
[2]: b26ebfe12f ("pid: Fix error return value in some cases")
[3]: 35f71bc0a0 ("fork: report pid reservation failure properly")
Cc: Corey Minyard <cminyard@mvista.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Adrian Reber <areber@redhat.com>
Cc: Dmitry Safonov <0x7f454c46@gmail.com>
Cc: Andrei Vagin <avagin@gmail.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

tools/testing/selftests/pidfd/pidfd.h

@@ -13,6 +13,8 @@
 #include <string.h>
 #include <syscall.h>
 #include <sys/mount.h>
+#include <sys/types.h>
+#include <sys/wait.h>
 
 #include "../kselftest.h"