Improve truncation_side (#14947)

* Enabling `truncation_side` for Slow and Fast tokenizer.

Co-Authored-by: Niels Rogge <48327001+NielsRogge@users.noreply.github.com>

* Disable failing tests.

* Layout xlm.

* assert -> assertEqual.

Co-authored-by: Niels Rogge <48327001+NielsRogge@users.noreply.github.com>

tests/test_tokenization_auto.py

@@ -209,6 +209,7 @@ class AutoTokenizerTest(unittest.TestCase):
         self.assertEqual(tokenizer.vocab_size, 30000)
         self.assertEqual(tokenizer.unk_token, "[UNK]")
         self.assertEqual(tokenizer.padding_side, "right")
+        self.assertEqual(tokenizer.truncation_side, "right")
 
     def test_auto_tokenizer_from_local_folder(self):
         tokenizer = AutoTokenizer.from_pretrained(SMALL_MODEL_IDENTIFIER)