[Circle ci] Install a virtual env before running tests
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Install a virtual env before running tests to not running into sudo issues when dynamically downloading files. 

Same number of tests now pass / fail as on my local computer: 
![Screenshot from 2020-05-01 12-14-44](https://user-images.githubusercontent.com/23423619/80798814-8a0a0a80-8ba5-11ea-8db8-599d33bbfccd.png)


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