tests: port pidfd_wait to kselftest harness

All of the new pidfd selftests already use the new kselftest harness
infrastructure. It makes for clearer output, makes the code easier to
understand, and makes adding new tests way simpler.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: linux-kselftest@vger.kernel.org
Link: https://lore.kernel.org/r/20200902102130.147672-4-christian.brauner@ubuntu.com