test_suite_pk: fix get_predefined_key_data() return value

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/tests/suites/test_suite_pk.function b/tests/suites/test_suite_pk.function
index 968f4dd..4d51c9e 100644
--- a/tests/suites/test_suite_pk.function
+++ b/tests/suites/test_suite_pk.function
@@ -204,7 +204,10 @@
         }
     }
 
-    return MBEDTLS_ERR_PK_BAD_INPUT_DATA;
+    TEST_FAIL("Unsupported key");
+    /* "exit" label is to make the compiler happy. */
+exit:
+    return MBEDTLS_ERR_PK_FEATURE_UNAVAILABLE;
 }
 
 #if defined(MBEDTLS_PSA_CRYPTO_C)