Is this number of "white-box" tests overkill? I see that even with 6 tests, I forgot to test the base case. It is a binary search tree insert function, from Module 6a of the Systematic Program Design MOOC by Gregor Kiczales.

insert-mysolution.rkt

(define BST0 false)
(define BST1 (make-node 1 "abc" false false))
(define BST4 (make-node 4 "dcj" false (make-node 7 "ruf" false false)))
(define BST3 (make-node 3 "ilk" BST1 BST4))
(define BST42 
  (make-node 42 "ily"
             (make-node 27 "wit" (make-node 14 "olp" false false) false)
             false))
(define BST10 (make-node 10 "why" BST3 BST42))

(check-expect (insert 43 "ygh" BST42)                    ; top-right
              (make-node 42 "ily"
                         (make-node 27 "wit"
                                    (make-node 14 "olp" false false)
                                    false)
                         (make-node 43 "ygh" false false)))
(check-expect (insert 2 "uvw" BST10)                     ; left-left-right
              (make-node 10 "why"
                         (make-node 3 "ilk"
                                    (make-node 1 "abc"
                                               false
                                               (make-node 2 "uvw" false false))
                                    BST4)
                         BST42)) ;
(check-expect (insert 37 "ijk" BST10)                     ; right-left-right
              (make-node 10 "why"
                         BST3
                         (make-node 42 "ily"
                                    (make-node 27 "wit"
                                               (make-node 14 "olp" false false)
                                               (make-node 37 "ijk" false false))
                                    false)))
(check-expect (insert 2 "uvw"
               (make-node 3 "ilk" false BST4))           ; top-left
              (make-node 3 "ilk"
                         (make-node 2 "uvw" false false)
                         BST4))
(check-expect (insert 5 "uvw"                            ; right-right-left
                      (make-node 3 "ilk"
                         (make-node 1 "abc" false false)
                         (make-node 6 "bcd"
                                    false
                                    (make-node 7 "ruf" false false))))                   
              (make-node 3 "ilk"
                         (make-node 1 "abc" false false)
                         (make-node 6 "bcd"
                                    (make-node 5 "uvw" false false)
                                    (make-node 7 "ruf" false false))))
(check-expect (insert 29 "hat"                            ; right-left-left
                      (make-node 10 "why"
                                 BST3
                                 (make-node 42 "ily"
                                            (make-node 35 "oxo" false false)
                                            (make-node 51 "bui" false false))))
              (make-node 10 "why"
                                 BST3
                                 (make-node 42 "ily"
                                            (make-node 35 "oxo" 
                                                       (make-node 29 "hat" false false)
                                                       false)
                                            (make-node 51 "bui" false false))))