Add some selftest testcases that validate the expected behavior of the
bpf_task_from_pid() kfunc that was added in the prior patch.
Signed-off-by: NDavid Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20221122145300.251210-3-void@manifault.comSigned-off-by: NAlexei Starovoitov <ast@kernel.org>