tests: fs: Add missing return check
tests: fs: Add missing return check

Checking the return of fs_stat to ensure that there is not hidden error.
Problem spotted by coverity.

CID 190949

Signed-off-by: Flavio Ceolin <979b9165500b0741b9d0500e2efd74fc1547bff7@intel.com>

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