selftests/bpf: Add tests for XDP bonding

Add a test suite to test XDP bonding implementation over a pair of
veth devices.

Signed-off-by: Jussi Maki <joamaki@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210731055738.16820-8-joamaki@gmail.com