centralize test helpers
Add funcarg to use the Examples from the Racket documentation directly.

Like that

    def test_string_copy(doctest):
        """
        > (define s1 "Yui")
        > (define pilot (string-copy s1))
        > (list s1 pilot)
        '("Yui" "Yui")
        > (for ([i (in-naturals)] [ch '(#\R #\e #\i)])
            (string-set! pilot i ch))
        > (list s1 pilot)
        '("Yui" "Rei")
        """
        assert doctest
Add 'substring for the strings benchmark (and small test)