Fix test_group_by_columns_single_column test in Tabular Test

[diegomarvid]

This test is not working

def test_group_by_columns_single_column() -> None:
    """Test to check if the group by columns is correctly applied."""
    dataset = input_data()
    data_container = DataContainer({"is_train": True})
    data_container.flow = dataset

    split_step = TabularSplitStep(train_percentage=0.7, group_by_columns=["category_low"])
    result = split_step.execute(data_container)

    assert isinstance(result, DataContainer)
    assert result.train.shape == (5, 7)
    assert result.validation.shape == (3, 7)
    assert len(result.train["category_low"].unique()) == 1
    assert len(result.validation["category_low"].unique()) == 1