selftests/bpf: add test for multiple programs

Add tests for having an XDP program attached in the driver and
another one attached in HW simultaneously.

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>