selftests/bpf: Improve inner_map test coverage.

Check that hash and array inner maps are properly initialized.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211201181040.23337-14-alexei.starovoitov@gmail.com