let-rec check (tests): fix the test for PR6939.
let-rec check (tests): fix the test for PR6939.

The unused variable warning is no longer emitted, since the code is
rejected at the typing stage by the new let-rec check.

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml