psa stats: Count locked slots instead of unlocked ones

Count locked slots and not unlocked ones to
align with the other statistics counters.

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/tests/include/test/psa_crypto_helpers.h b/tests/include/test/psa_crypto_helpers.h
index 09171ae..01b0547 100644
--- a/tests/include/test/psa_crypto_helpers.h
+++ b/tests/include/test/psa_crypto_helpers.h
@@ -41,7 +41,7 @@
         msg = "An external slot has not been closed properly.";
     else if( stats.half_filled_slots != 0 )
         msg = "A half-filled slot has not been cleared properly.";
-    else if( stats.unlocked_slots != PSA_KEY_SLOT_COUNT )
+    else if( stats.locked_slots != 0 )
     {
         msg = "Some slots are still marked as locked.";
     }