selftests: pidfd: Fix compling warnings

Fix warnings and enable Wall.

pidfd_wait.c: In function ‘wait_nonblock’:
pidfd_wait.c:150:13: warning: unused variable ‘status’ [-Wunused-variable]
  150 |  int pidfd, status = 0;
      |             ^~~~~~
...
pidfd_test.c: In function ‘child_poll_exec_test’:
pidfd_test.c:438:1: warning: no return statement in function returning non-void [-Wreturn-type]
  438 | }
      | ^

Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
v2: fix mistake assignment to pidfd
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>