tests: add waitid() tests for non-blocking pidfds

Verify that the PIDFD_NONBLOCK flag works with pidfd_open() and that
waitid() with a non-blocking pidfd returns EAGAIN:

	TAP version 13
	1..3
	# Starting 3 tests from 1 test cases.
	#  RUN           global.wait_simple ...
	#            OK  global.wait_simple
	ok 1 global.wait_simple
	#  RUN           global.wait_states ...
	#            OK  global.wait_states
	ok 2 global.wait_states
	#  RUN           global.wait_nonblock ...
	#            OK  global.wait_nonblock
	ok 3 global.wait_nonblock
	# PASSED: 3 / 3 tests passed.
	# Totals: pass:3 fail:0 xfail:0 xpass:0 skip:0 error:0

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/20200902102130.147672-5-christian.brauner@ubuntu.com