Improve CI performance speed of PackagedDatasetTest 
Author: albertvillanova | Pull request: yes | State: closed

This PR improves PackagedDatasetTest CI performance speed. For Ubuntu (latest):
- Duration (without parallelism) before: 334.78s (5.58m)
- Duration (without parallelism) afterwards: 0.48s

The approach is passing a dummy `data_files` argument to load the builder, so that it avoids the slow inferring of it over the entire root directory of the repo.

## Total duration of PackagedDatasetTest
| | Before | Afterwards | Improvement
|---|---:|---:|---:|
| Linux | 334.78s | 0.48s | x700
| Windows | 513.02s | 1.09s | x500

## Durations by each individual sub-test
More accurate durations, running them on GitHub, for Linux (latest).

Before this PR, the total test time (without parallelism) for `tests/test_dataset_common.py::PackagedDatasetTest` is 334.78s (5.58m)
```
39.07s call     tests/test_dataset_common.py::PackagedDatasetTest::test_load_dataset_offline_imagefolder
38.94s call     tests/test_dataset_common.py::PackagedDatasetTest::test_load_dataset_offline_audiofolder
34.18s call     tests/test_dataset_common.py::PackagedDatasetTest::test_load_dataset_offline_parquet
34.12s call     tests/test_dataset_common.py::PackagedDatasetTest::test_load_dataset_offline_csv
34.00s call     tests/test_dataset_common.py::PackagedDatasetTest::test_load_dataset_offline_pandas
34.00s call     tests/test_dataset_common.py::PackagedDatasetTest::test_load_dataset_offline_text
33.86s call     tests/test_dataset_common.py::PackagedDatasetTest::test_load_dataset_offline_json
10.39s call     tests/test_dataset_common.py::PackagedDatasetTest::test_builder_class_audiofolder
6.50s call     tests/test_dataset_common.py::PackagedDatasetTest::test_builder_configs_audiofolder
6.46s call     tests/test_dataset_common.py::PackagedDatasetTest::test_builder_configs_imagefolder
6.40s call     tests/test_dataset_common.py::PackagedDatasetTest::test_builder_class_imagefolder
5.77s call     tests/test_dataset_common.py::PackagedDatasetTest::test_builder_class_csv
5.77s call     tests/test_dataset_common.py::PackagedDatasetTest::test_builder_class_text
5.74s call     tests/test_dataset_common.py::PackagedDatasetTest::test_builder_configs_parquet
5.69s call     tests/test_dataset_common.py::PackagedDatasetTest::test_builder_class_json
5.68s call     tests/test_dataset_common.py::PackagedDatasetTest::test_builder_configs_pandas
5.67s call     tests/test_dataset_common.py::PackagedDatasetTest::test_builder_class_parquet
5.67s call     tests/test_dataset_common.py::PackagedDatasetTest::test_builder_class_pandas
5.66s call     tests/test_dataset_common.py::PackagedDatasetTest::test_builder_configs_json
5.66s call     tests/test_dataset_common.py::PackagedDatasetTest::test_builder_configs_csv
5.55s call     tests/test_dataset_common.py::PackagedDatasetTest::test_builder_configs_text

(42 durations < 0.005s hidden.)
```

With this PR: 0.48s
```
0.09s call     tests/test_dataset_common.py::PackagedDatasetTest::test_load_dataset_offline_audiofolder
0.08s call     tests/test_dataset_common.py::PackagedDatasetTest::test_load_dataset_offline_csv
0.08s call     tests/test_dataset_common.py::PackagedDatasetTest::test_load_dataset_offline_imagefolder
0.06s call     tests/test_dataset_common.py::PackagedDatasetTest::test_load_dataset_offline_json
0.05s call     tests/test_dataset_common.py::PackagedDatasetTest::test_builder_class_audiofolder
0.05s call     tests/test_dataset_common.py::PackagedDatasetTest::test_load_dataset_offline_parquet
0.04s call     tests/test_dataset_common.py::PackagedDatasetTest::test_load_dataset_offline_pandas
0.03s call     tests/test_dataset_common.py::PackagedDatasetTest::test_load_dataset_offline_text

(55 durations < 0.005s hidden.)
```