[Refactor] mark build_dataflow_dir test as end2end

tests/util/test_build_dataflow.py

@@ -39,7 +39,7 @@ from finn.util.basic import make_build_dir
 
 @pytest.mark.slow
 @pytest.mark.vivado
-def test_build_dataflow_directory():
+def test_end2end_build_dataflow_directory():
     test_dir = make_build_dir("test_build_dataflow_directory_")
     target_dir = test_dir + "/build_dataflow"
     example_data_dir = pk.resource_filename("finn.qnn-data", "build_dataflow/")