tests: net: net_pkt: Verify unchecked net_pkt functions results
tests: net: net_pkt: Verify unchecked net_pkt functions results

Some net_pkt functions in `test_net_pkt_headroom_copy()` did not have
their return values verified, causing coverity complains.

CID: 240661

Fixes #39841

Signed-off-by: Robert Lubos <9d28997c6894a2d0321a7bb399169af7ce99b64e@nordicsemi.no>

galak/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr