selftests/bpf: Add test for sleepable bpf iterator programs

This adds a test for bpf iterator programs to make use of sleepable
bpf helpers.

Signed-off-by: Kenny Yu <kennyyu@fb.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220124185403.468466-5-kennyyu@fb.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>