Convert a bunch of selftests to using newly added high-level BPF map
APIs.

This change exposed that map_kptr selftests allocated too big buffer,
which is fixed in this patch as well.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220512220713.2617964-2-andrii@kernel.org