test: mark two flaky test (due to race condition) as serial (#5918)

Author: lucasjia-aws

sagemaker-train/tests/integ/train/test_custom_scorer_evaluator.py

@@ -246,6 +246,7 @@ def test_custom_scorer_evaluator_validation(self):
 
     # @pytest.mark.skip(reason="Built-in metric evaluation - to be enabled when needed")
     @pytest.mark.gpu_intensive
+    @pytest.mark.serial
     def test_custom_scorer_with_builtin_metric(self):
         """
         Test custom scorer evaluation with built-in metric.
@@ -299,6 +300,7 @@ def test_custom_scorer_with_builtin_metric(self):
 
     # @pytest.mark.skip(reason="Base model only evaluation - not working yet per notebook")
     @pytest.mark.gpu_intensive
+    @pytest.mark.serial
     def test_custom_scorer_base_model_only(self):
         """
         Test custom scorer evaluation with base model only (no fine-tuned model).