Run tests in parallel
MEMBER
There is a speed up in Windows machines:
- From `13m 52s` to `11m 10s`

In Linux machines, some workers crash with error message:
```
OSError: [Errno 12] Cannot allocate memory
```
2021-09-22T07:24:50Z
https://github.com/huggingface/datasets/pull/2954#issuecomment-924658896
https://api.github.com/repos/huggingface/datasets/issues/2954
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2021-09-22T07:24:50Z
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MEMBER
There is also a speed up in Linux machines:
- From `7m 30s` to `5m 32s`
2021-09-22T07:52:54Z
https://github.com/huggingface/datasets/pull/2954#issuecomment-924676528
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2021-09-22T07:52:54Z
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Run CI tests in parallel to speed up the test suite.

Speed up results:
- Linux: from `7m 30s` to `5m 32s`
- Windows: from `13m 52s` to `11m 10s`
