selftests/bpf: Add netkit tests for mac address

This adds simple tests around setting MAC addresses in the different
netkit modes.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20240524163619.26001-3-daniel@iogearbox.net


Signed-off-by: Alexei Starovoitov <ast@kernel.org>