tests: subsys: logging: move log_backend_init to new ztest API
tests: subsys: logging: move log_backend_init to new ztest API

Fix missing to https://github.com/zephyrproject-rtos/zephyr/issues/49004

Signed-off-by: li biao <278ffa4c2e907b9ec147f466cdc373397e29446b@intel.com>

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