Add strain exercise (#297)

[Adrien-LUDWIG]

Are these two lambda acceptable? These are the predicates given to the tested function.
For the latter, is it ok to use list in this case? The lambda doesn't work with the quote form.

     (test-equal? "keeps strings"
                  (keep '("apple" "zebra" "banana" "zombies" "cherimoya" "zealot")
                        (lambda (x) (equal? (string-ref x 0) #\z)))
                  '("zebra" "zombies" "zealot"))

     (test-equal? "keeps lists"
                  (keep (list '(1 2 3) '(5 5 5) '(5 1 2) '(2 1 2) '(1 5 2) '(2 2 1) '(1 2 5))
                        (lambda (x) (if (member 5 x) #t #f)))
                  (list '(5 5 5) '(5 1 2) '(1 5 2) '(1 2 5)))

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[Adrien-LUDWIG]

@BNAndras

It's a ping again. 😅

[Adrien-LUDWIG]

Oh wow, sorry. It's not very DRY. I will fix these right now.

[Adrien-LUDWIG]

It's cleaned.

Thank you for the time and effort you put into these reviews! 🚀