test: net: ipv6_fragment: Set link layer protocol type for a packet

The test_send_ipv6_fragment_without_hbho() test did not set link layer
protocol type for the packet. It is needed in this case because we
are sending a ready made packet and the protocol type is unknown
otherwise.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>