selftests/bpf: Add access_inner_map selftest

Add a selftest that accesses a BPF_MAP_TYPE_ARRAY (at a nonzero index)
nested within a BPF_MAP_TYPE_HASH_OF_MAPS to flex a previously buggy
case.

Signed-off-by: Rhys Rustad-Elliott <me@rhysre.net>
Link: https://lore.kernel.org/r/20230602190110.47068-3-me@rhysre.net
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>