[Tests] test only for fast test as a default
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Test failure is not related to change in test file.

2020-05-05T13:01:57Z
https://github.com/huggingface/datasets/pull/50#issuecomment-624041328
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2020-05-05T13:01:57Z
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Test only for one config on circle ci to speed up testing. Add all config test as a slow test. 
@mariamabarham @thomwolf 