So, two things I missed:
New Test
In my template code, I missed a test case. Now, you could have (perhaps) caught this yourself. (My tests were not promised to be complete.) You might want to consider adding the following to your "flatten" test suite:
(check-equal? (list 1 2 3 4 5)
(flatten (list (list 1 (list 2))
(list 3 (list 4 (list) (list 5)))))
"Tierney's case.")
This is called "Tierney's case" because it was her code that illustrated the fact that my test suite could have been more complete. (If everything is/was working for you, then adding this test to your test suite shouldn't matter. Up to you.)
New Test Function
Again, I honestly thought this was in your template code. You may have noticed that you cannot compare two structures in Scheme with equal?. I've posted some code that will compare two trees. Take a look at binary-tree-check.ss if you want to be able to compare trees in your test suite.