Do export sanity checks in import_export as well

This is not useful to validate the implementation when importing
canonical input, which is the case for most import/export test cases,
but it helps validate the sanity checks themselves.

tests/suites/test_suite_psa_crypto.function

@@ -1001,6 +1001,9 @@ void import_export( data_t *data,
         goto destroy;
     }
 
+    if( ! exercise_export_key( slot, usage_arg ) )
+        goto exit;
+
     if( canonical_input )
     {
         TEST_ASSERT( exported_length == data->len );