Fix mod-test-sum-test for bignums

* test/src/emacs-module-tests.el (mod-test-sum-test): Update
for bignums.

test/src/emacs-module-tests.el

@@ -68,10 +68,10 @@
                (1+ #x1fffffff)))
     (should (= (mod-test-sum -1 (1+ #x1fffffff))
                #x1fffffff)))
-  (should-error (mod-test-sum 1 most-positive-fixnum)
-                :type 'overflow-error)
-  (should-error (mod-test-sum -1 most-negative-fixnum)
-                :type 'overflow-error))
+  (should (= (mod-test-sum 1 most-positive-fixnum)
+             (1+ most-positive-fixnum)))
+  (should (= (mod-test-sum -1 most-negative-fixnum)
+             (1- most-negative-fixnum))))
 
 (ert-deftest mod-test-sum-docstring ()
   (should (string= (documentation 'mod-test-sum) "Return A + B\n\n(fn a b)")))