tests: net: arp: Fix net_buf assert in ARP unit test

Fix ASSERTs that appears when enabling asserts in net buf.
Asserts are:
 - Pulling from net buf before any data has been added.
 - Pulling more data than has been allocated for the buf.

Fix warning:
W: You have 1 IPv4 net_if addresses but 2 network interfaces
W: Consider increasing CONFIG_NET_IF_MAX_IPV4_COUNT value.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>
2 files changed