tests: timer: fix improper test identifier
tests: timer: fix improper test identifier

The test identifier of timer error case in testcase.yaml is
exactly the same as timer api's test identifier.
So I fix this.

Signed-off-by: Lixin Guo <8de84fecf2daa12e9dd27dd0d7f47ee7775a73c0@intel.com>

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