fix a test

Using `racket` now creates a will executor via `ffi/unsafe`, which
interferes with the test's expected count, while using `racket/base`
doesn't.

pkgs/racket-test/tests/racket/prompt-sfs.rkt

@@ -1,4 +1,4 @@
-#lang racket
+#lang racket/base
 (require racket/system)
 
 #|