tests: subsys: debug: cpu_load: Change duplicated test name

Fix Twister runtime error.
twisterlib.error.TwisterRuntimeError: Duplicated test scenarios found:
- debug.cpu_load found in:
./ncs/zephyr/tests/subsys/debug/cpu_load
./ncs/nrf/tests/subsys/debug/cpu_load

Signed-off-by: Sebastian Głąb <[email protected]>

Author: nordic-segl

tests/subsys/debug/cpu_load/testcase.yaml

@@ -1,5 +1,5 @@
 tests:
-  debug.cpu_load:
+  debug.cpu_load.nrf:
     sysbuild: true
     platform_allow:
       - nrf52840dk/nrf52840