psa: slot mgmt: Add unaccessed slots counter in stats

Add a counter of unaccessed slots and use it
in tests to check that at the end of PSA tests
all key slot are unaccessed.

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/library/psa_crypto_slot_management.c b/library/psa_crypto_slot_management.c
index e207477..2fa0a0d 100644
--- a/library/psa_crypto_slot_management.c
+++ b/library/psa_crypto_slot_management.c
@@ -408,6 +408,10 @@
     for( slot_idx = 0; slot_idx < PSA_KEY_SLOT_COUNT; slot_idx++ )
     {
         const psa_key_slot_t *slot = &global_data.key_slots[ slot_idx ];
+        if( ! psa_is_key_slot_accessed( slot ) )
+        {
+            ++stats->unaccessed_slots;
+        }
         if( ! psa_is_key_slot_occupied( slot ) )
         {
             ++stats->empty_slots;