selftests: net: veth: test the ability to independently manipulate GRO and XDP

We should be able to independently flip either XDP or GRO states and toggling
one should not affect the other.

Adjust other tests as well that had implicit expectation that GRO would be
automatically enabled.

Signed-off-by: Ignat Korchagin <ignat@cloudflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>