Get a builtin key's attributes in order to correctly get its size

Leverage the fact that the get_builtin_key entrypoint returns a key's
attributes, such that a proper size for the builtin key's buffer can
be calculated through the driver's get_key_buffer_size hook.

Signed-off-by: Steven Cooreman <steven.cooreman@silabs.com>
diff --git a/tests/src/drivers/key_management.c b/tests/src/drivers/key_management.c
index 5daec6b..a0626fb 100644
--- a/tests/src/drivers/key_management.c
+++ b/tests/src/drivers/key_management.c
@@ -343,9 +343,6 @@
     psa_key_attributes_t *attributes,
     uint8_t *key_buffer, size_t key_buffer_size, size_t *key_buffer_length )
 {
-    if( key_buffer_size < sizeof( psa_drv_slot_number_t ) )
-        return( PSA_ERROR_BUFFER_TOO_SMALL );
-
     switch( slot_number )
     {
         case PSA_CRYPTO_TEST_DRIVER_BUILTIN_AES_KEY_SLOT:
@@ -358,6 +355,9 @@
                 PSA_KEY_USAGE_EXPORT );
             psa_set_key_algorithm( attributes, PSA_ALG_CTR );
 
+            if( key_buffer_size < sizeof( psa_drv_slot_number_t ) )
+                return( PSA_ERROR_BUFFER_TOO_SMALL );
+
             *( (psa_drv_slot_number_t*) key_buffer ) =
                 PSA_CRYPTO_TEST_DRIVER_BUILTIN_AES_KEY_SLOT;
             *key_buffer_length = sizeof( psa_drv_slot_number_t );
@@ -375,6 +375,9 @@
             psa_set_key_algorithm(
                 attributes, PSA_ALG_ECDSA( PSA_ALG_ANY_HASH ) );
 
+            if( key_buffer_size < sizeof( psa_drv_slot_number_t ) )
+                return( PSA_ERROR_BUFFER_TOO_SMALL );
+
             *( (psa_drv_slot_number_t*) key_buffer ) =
                 PSA_CRYPTO_TEST_DRIVER_BUILTIN_ECDSA_KEY_SLOT;
             *key_buffer_length = sizeof( psa_drv_slot_number_t );