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>
diff --git a/tests/subsys/llext/simple/testcase.yaml b/tests/subsys/llext/simple/testcase.yaml
index 961d94f..9f9f70a 100644
--- a/tests/subsys/llext/simple/testcase.yaml
+++ b/tests/subsys/llext/simple/testcase.yaml
@@ -50,9 +50,8 @@
   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