Test slot_number attribute

Test the behavior of the getter/setter functions.

Test that psa_get_key_slot_number() reports a slot number for a key in
a secure element, and doesn't report a slot number for a key that is
not in a secure element.

Test that psa_get_key_slot_number() reports the correct slot number
for a key in a secure element.
diff --git a/include/psa/crypto_extra.h b/include/psa/crypto_extra.h
index 5359b58..130ce75 100644
--- a/include/psa/crypto_extra.h
+++ b/include/psa/crypto_extra.h
@@ -163,6 +163,18 @@
     attributes->slot_number = slot_number;
 }
 
+/** Remove the slot number attribute from a key attribute structure.
+ *
+ * This function undoes the action of psa_set_key_slot_number().
+ *
+ * \param[out] attributes       The attribute structure to write to.
+ */
+static inline void psa_clear_key_slot_number(
+    psa_key_attributes_t *attributes )
+{
+    attributes->core.flags &= ~MBEDTLS_PSA_KA_FLAG_HAS_SLOT_NUMBER;
+}
+
 #endif /* MBEDTLS_PSA_CRYPTO_SE_C */
 
 /**@}*/