testing/racket: upgrade to 8.2

Merged Sören Tempel requested to merge nmeum/aports:chez-scheme-8.1 into master

This also switches from Racket BC to Racket CS (chez-scheme). For this reason, it no longer uses the bundled libffi. It does, however, use a bundled version of chez-scheme which should (if possible) be replaced with our packaged version in future updates.

Edited by Andy Postnikov

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