Add global fixtures for dummy datasets
Add global fixtures for dummy datasets

This allow accessing pre-generated builder in test without having to generate them for each tests. Builders are shared across all tests.

```
def test_xyz(dummy_dataset: tfds.testing.DummyDataset):
  ...
```

PiperOrigin-RevId: 379233178

tensorflow/datasets,tensorflow/datasets,tensorflow/datasets,tensorflow/datasets,tensorflow/datasets