test: net: net_pkt: use var buffer pool for testing

From change 47eb592c28b9e7dfbdd25fedbf07a528ad240084 net_buf structures
allocated from pools defined with NET_BUF_POOL_FIXED_DEFINE() will keep
their `size` member set to the actual fixed size and not requested
size like before.

For this set of tests NET_BUF_POOL_FIXED_DEFINE() for several test
cases. These tests check the several values based on a fact that the
network buffer's size was set to the requested size.

This commit changes definition of the buffer pool to
NET_BUF_POOL_FIXED_DEFINE() in order to satisfy these expectations.
This change does not impact the tests themselves.

Signed-off-by: Konrad Derda <konrad.derda@nordicsemi.no>