tests: net: arp: Increase network buffer counts

In some hw like Atmel SAM-E70, the original amount of network
buffers is too small so increasing it slightly.
Also generate proper error if this happens in the test so it
is easier to figure out.

Fixes #8866

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>