tests: zephyr: drivers: adc: adc_accuracy_test: use fixture

To select board with needed loopback.

Signed-off-by: Piotr Kosycarz <piotr.kosycarz@nordicsemi.no>

Author: nordic-piks

tests/zephyr/drivers/adc/adc_accuracy_test/testcase.yaml

@@ -4,8 +4,8 @@ common:
     - ci_tests_zephyr_drivers_adc
 tests:
   nrf.extended.drivers.adc.accuracy.ref_volt:
-    # harness_config:
-    #   fixture: adc_ref_volt
+    harness_config:
+      fixture: adc_ref_volt
     integration_platforms:
       - nrf54ls05dk/nrf54ls05b/cpuapp
     platform_allow: