disable embed-in-C test

Added to the release checklist, instead, since it requires a
`configure`-time flag.

pkgs/racket-pkgs/racket-test/tests/racket/embed-in-c.rkt

@@ -1,9 +1,12 @@
 #lang racket
 
-;; Works for Linux.
+;; Works for Linux when the build is configured `--enable-libs`.
 ;; Almost works for Mac OS X.
 ;; Assumes 3m.
 
+;; Since `--enable-libs` is not the default, disable test for now:
+(module test racket/base)
+
 (require racket/system
          setup/dirs)