test: drivers: clock_control_api: Fix conf file application

CONF_FILE parameter in testcase.yaml overwrites board
specific configurations. Change it to EXTRA_CONF_FILE
to apply both configurations.

Signed-off-by: Michał Stasiak <michal.stasiak@nordicsemi.no>

Author: mstasiaknordic

tests/drivers/clock_control/clock_control_api/testcase.yaml

@@ -38,4 +38,4 @@ tests:
       - ophelia4ev/nrf54l15/cpuapp
     integration_platforms:
       - nrf51dk/nrf51822
-    extra_args: CONF_FILE="nrf_lfclk_rc.conf"
+    extra_args: EXTRA_CONF_FILE="nrf_lfclk_rc.conf"