[testing] set tests to not rebuild datasets
recently `datasets` created in memory datasets enabled by default - which is great for those who wants it but is a terrible idea for tests and those who need to develop things constantly restarting the scripts as datasets aren't being cached and rebuilt on every run.

So we should turn this feature off in `*/conftest.py` by setting:
```
HF_MAX_IN_MEMORY_DATASET_SIZE_IN_BYTES=0 
```

But it's going to be renamed shortly to `HF_DATASETS_IN_MEMORY_MAX_SIZE`
https://github.com/huggingface/datasets/pull/2409#issuecomment-850549742
https://github.com/huggingface/datasets/pull/2454

So for now this issue is tracking this change and then will add it to the tests.


`datasets` has just made this feature disabled by default: https://github.com/huggingface/datasets/pull/2460

So nothing needs to be done.