Ignore ert-deftest body value

* lisp/emacs-lisp/ert.el (ert-deftest):
Since the return value of the body isn't going to be used, ignore
it explicitly so that the compiler can warn if we try to return
something anyway.  In particular, this exposes some comparisons
whose result weren't actually checked.

lisp/emacs-lisp/ert.el

@@ -237,7 +237,9 @@ in batch mode, an error is signaled.
                             `(:expected-result-type ,expected-result))
                         ,@(when tags-supplied-p
                             `(:tags ,tags))
-                        :body (lambda () ,@body)
+                        ;; Add `nil' after the body to enable compiler warnings
+                        ;; about unused computations at the end.
+                        :body (lambda () ,@body nil)
                         :file-name ,(or (macroexp-file-name) buffer-file-name)))
          ',name))))