Fix parallel tests `make check -j`
Fix parallel tests `make check -j`

Summary:
parallel tests are broken because gnu_parallel is reading deprecated options from `/etc/parallel/config`
Fix this by passing `--plain` to ignore `/etc/parallel/config`

Test Plan: make check -j64

Reviewers: kradhakrishnan, sdong, andrewkr, yiwu, arahut

Reviewed By: arahut

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D61359

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