selftests/bpf: add test for sharing objects between netdevs

Add tests for sharing programs and maps between different netdevs.
Use netdevsim's ability to pretend multiple netdevs belong to the
same "ASIC".

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>