Adds missing module_specs for usages of _LazyModule (#15230)

* Add missing __spec__ for transformers.models.auto

* Moves the __spec__-test to the UnitTest class

* Adds module_spec to all instances of _LazyModule

* Refactors an old test from pytest to unittest

tests/test_file_utils.py

@@ -64,9 +64,11 @@ def context_fr():
     print("Au revoir!")
 
 
-def test_module_spec():
-    assert transformers.__spec__ is not None
-    assert importlib.util.find_spec("transformers") is not None
+class TestImportMechanisms(unittest.TestCase):
+    def test_module_spec_available(self):
+        # If the spec is missing, importlib would not be able to import the module dynamically.
+        assert transformers.__spec__ is not None
+        assert importlib.util.find_spec("transformers") is not None
 
 
 class GetFromCacheTests(unittest.TestCase):