tests: shell: Fix string not null-terminated
tests: shell: Fix string not null-terminated

shell_exec expects a string that is null-terminated but if line passed
to strncpy is equal or bigger than the buffer it will not produce a
null-terminated command.

Jira: ZEP-2474

Signed-off-by: Luiz Augusto von Dentz <8530c5ea66a1bdbc08f98bfcf183c8be901b5990@intel.com>

rsalveti/zephyr,nashif/zephyr,aceofall/zephyr-iotos,nashif/zephyr,zephyrproject-rtos/zephyr,rsalveti/zephyr,aceofall/zephyr-iotos,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,kraj/zephyr,Vudentz/zephyr,explora26/zephyr,nashif/zephyr,kraj/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,fbsder/zephyr,runchip/zephyr-cc3220,ldts/zephyr,aceofall/zephyr-iotos,finikorg/zephyr,runchip/zephyr-cc3220,galak/zephyr,punitvara/zephyr,Vudentz/zephyr,finikorg/zephyr,explora26/zephyr,explora26/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,ldts/zephyr,fbsder/zephyr,GiulianoFranchetto/zephyr,zephyriot/zephyr,Vudentz/zephyr,runchip/zephyr-cc3220,rsalveti/zephyr,galak/zephyr,Vudentz/zephyr,ldts/zephyr,runchip/zephyr-cc3220,kraj/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,explora26/zephyr,zephyriot/zephyr,fbsder/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,rsalveti/zephyr,zephyriot/zephyr,punitvara/zephyr,runchip/zephyr-cc3220,zephyriot/zephyr,galak/zephyr,mbolivar/zephyr,mbolivar/zephyr,ldts/zephyr,aceofall/zephyr-iotos,mbolivar/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,rsalveti/zephyr,fbsder/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,zephyriot/zephyr,punitvara/zephyr,aceofall/zephyr-iotos,Vudentz/zephyr,kraj/zephyr,fbsder/zephyr,explora26/zephyr,galak/zephyr