Tag tests as slow ⌛ (#21537)

begone slow tests

tests/models/mobilebert/test_modeling_tf_mobilebert.py

@@ -317,6 +317,11 @@ class TFMobileBertModelTest(TFModelTesterMixin, unittest.TestCase):
                 name = model.get_bias()
                 assert name is None
 
+    @slow
+    def test_keras_fit(self):
+        # Override as it is a slow test on this model
+        super().test_keras_fit()
+
     @tooslow
     def test_saved_model_creation(self):
         pass