llext: set integration platform on relocatable test

This particular test was ignored in CI because, when using
platform_key, Twister picks the specific platform very early in
the process and it is possible that the chosen one is later
filtered out. To fix this, use the `integration_platforms` key
instead to specify the platforms where the test is expected to
complete successfully.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>

tests/subsys/llext/simple/testcase.yaml

@@ -50,9 +50,8 @@ tests:
   llext.simple.modules_enabled_writable_relocatable:
     arch_exclude: arm arm64
     filter: not CONFIG_MPU and not CONFIG_MMU
-    platform_key:
-      - simulation
-      - arch
+    integration_platforms:
+      - qemu_xtensa
     extra_configs:
       - CONFIG_MODULES=y
       - CONFIG_LLEXT_STORAGE_WRITABLE=y