selftests/bpf: Improve percpu_alloc test robustness

Make these non-serial tests filter BPF programs by intended PID of
a test runner process. This makes it isolated from other parallel tests
that might interfere accidentally.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20231011223728.3188086-2-andrii@kernel.org