[testing] slow tests should be marked as slow (#7895)

* slow tests should be slow

* exception note

* style

* integrate LysandreJik's notes with some expansions

* Apply suggestions from code review

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* another slow test

* fix link, and prose

* clarify.

* note from Sam

* typo

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

tests/test_modeling_marian.py

@@ -295,6 +295,7 @@ class TestMarian_en_ROMANCE(MarianIntegrationTest):
         with self.assertRaises(ValueError):
             self.tokenizer.prepare_seq2seq_batch([""])
 
+    @slow
     def test_pipeline(self):
         device = 0 if torch_device == "cuda" else -1
         pipeline = TranslationPipeline(self.model, self.tokenizer, framework="pt", device=device)