tests: fix unchecked return value in test_net_pkt_basics_of_rw
tests: fix unchecked return value in test_net_pkt_basics_of_rw

Inside function test_net_pkt_basics_of_rw result of function call
net_pkt_read_be16 is not checked which might result performance
of the program and cause errors.

Coverity-CID 198003
Fixes #15776

Signed-off-by: Maksim Masalski <4eb3679da23cf41c68efaea5c81c9e87b8df5f95@gmail.com>

GiulianoFranchetto/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,ldts/zephyr,nashif/zephyr,ldts/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,ldts/zephyr,galak/zephyr,ldts/zephyr,galak/zephyr