selftests/bpf: extend existing map resize tests for per-cpu use case

Add a per-cpu array resizing use case and demonstrate how
bpf_get_smp_processor_id() can be used to directly access proper data
with no extra checks.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230711232400.1658562-2-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>