selftests: net: bridge: igmp: check for specific udp ip protocol

We have to specifically check for udp protocol in addition to the mac
address because in IGMPv3 tests group-specific queries will use the same
mac address.

Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>