tests: zephyr: drivers: flash: add testcase for single

Since snippets is now using quad mode by default,
separate testcase should be added to keep testing
single mode.

Signed-off-by: Marcin Szymczyk <marcin.szymczyk@nordicsemi.no>

Author: masz-nordic

tests/zephyr/drivers/flash/common/single.overlay

@@ -0,0 +1,3 @@
+&mx25r64 {
+	mspi-io-mode = "MSPI_IO_MODE_SINGLE";
+};

tests/zephyr/drivers/flash/common/testcase.yaml

@@ -4,8 +4,14 @@ common:
     - flash
     - ci_tests_drivers_sdp
 tests:
-  nrf.extended.drivers.flash.common.sdp:
+  nrf.extended.drivers.flash.common.sdp.quad:
     platform_allow:
       - nrf54l15dk/nrf54l15/cpuapp
     integration_platforms:
       - nrf54l15dk/nrf54l15/cpuapp
+  nrf.extended.drivers.flash.common.sdp.single:
+    platform_allow:
+      - nrf54l15dk/nrf54l15/cpuapp
+    integration_platforms:
+      - nrf54l15dk/nrf54l15/cpuapp
+    extra_args: EXTRA_DTC_OVERLAY_FILE="single.overlay"