add test folder

tests/test_fetch_series_by_api_link.m

tests/test_fetch_series_by_code.m

tests/test_fetch_series_by_code_mask.m

...t_fetch_series_by_code_mask_with_plus_in_dimension_code.m

tests/test_fetch_series_by_dimension.m

tests/test_fetch_series_by_id.m

tests/test_fetch_series_by_ids_in_different_datasets.m

tests/test_fetch_series_by_ids_in_same_dataset.m

tests/test_fetch_series_of_dataset.m

tests/test_fetch_series_with_max_nb_series.m

tests/test_fetch_series_with_na_values.m