tests: drivers: flash: common: require external_flash fixture

Add "external_flash" fixture to the no_explicit_erase testcase
in order to run twister only on boards with such HW setup.
Other runnable test cases has it already.

Signed-off-by: Piotr Kosycarz <[email protected]>
(cherry picked from commit 2318832)

Author: nordic-piks

tests/drivers/flash/common/testcase.yaml

@@ -60,7 +60,8 @@ tests:
       - nrf54l15dk/nrf54l10/cpuapp
       - nrf54l15dk/nrf54l15/cpuapp
       - nrf54h20dk/nrf54h20/cpuapp
-      - ophelia4ev/nrf54l15/cpuapp
+    harness_config:
+      fixture: external_flash
   drivers.flash.common.nrf54lm20a:
     platform_allow:
       - nrf54lm20dk/nrf54lm20a/cpuapp