Test iterators of vma, files and tasks.

Ensure the API works appropriately to visit all tasks,
tasks in a process, or a particular task.
Signed-off-by: NKui-Feng Lee <kuifeng@fb.com>
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Acked-by: NYonghong Song <yhs@fb.com>
Acked-by: NMartin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/bpf/20220926184957.208194-5-kuifeng@fb.com