Improve test method name

Co-authored-by: Merel Theisen <49397448+merelcht@users.noreply.github.com>

kedro-datasets/tests/polars/test_generic_dataset.py

@@ -114,7 +114,7 @@ def excel_data_set_overwrite(dummy_dataframe: pl.DataFrame, filepath_excel):
 
 
 class TestGenericExcelDataSet:
-    def test_assert_write_mode(self):
+    def test_assert_write_mode_fails(self):
         pattern = (
             "Write mode `test` is not supported. "
             "Allowed values are: overwrite, ignore."