tests: mspi: flash: Use platform_allow for the no-multithreading test

Since many boards enable features that require multithreading (e.g. USB)
that conflict directly with the `CONFIG_MULTITHREADING=n` required for
the test variant, and the filter:
filter: dt_alias_exists("mspi0") and CONFIG_FLASH_MSPI_NOR
does not work since that requires Devicetree and Kconfig building
successfully, which is not the case here (Kconfig fails since
dependencies are not met).

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

Author: carlescufi

tests/drivers/mspi/flash/testcase.yaml

@@ -38,7 +38,7 @@ tests:
       - CONFIG_MSPI_DW_HANDLE_FIFOS_IN_SYSTEM_WORKQUEUE=y
   drivers.mspi.flash.mspi_nor_no_multithreading:
     filter: dt_alias_exists("mspi0") and CONFIG_FLASH_MSPI_NOR
-    integration_platforms:
+    platform_allow:
       - nrf54h20dk/nrf54h20/cpuapp
     extra_configs:
       - CONFIG_MULTITHREADING=n