selftests/bpf: Add selftests for bpf_task_from_pid()

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: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20221122145300.251210-3-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>