Add an expensive test for defcustom types

* admin/cus-test.el (cus-test-opts): Return the tests.

* test/lisp/custom-tests.el (check-for-wrong-custom-types): Test
custom types (bug#30990).

admin/cus-test.el

@@ -370,7 +370,9 @@ This function is suitable for batch mode.  E.g., invoke
 
 in the Emacs source directory.
 Normally only tests options belonging to files in loaddefs.el.
-If optional argument ALL is non-nil, test all files with defcustoms."
+If optional argument ALL is non-nil, test all files with defcustoms.
+
+Returns a list of variables with suspicious types."
   (interactive)
   (and noninteractive
        command-line-args-left
@@ -382,9 +384,12 @@ If optional argument ALL is non-nil, test all files with defcustoms."
   (message "Running %s" 'cus-test-apropos)
   (cus-test-apropos "")
   (if (not cus-test-errors)
-      (message "No problems found")
+      (progn
+        (message "No problems found")
+        nil)
     (message "The following options might have problems:")
-    (cus-test-message cus-test-errors)))
+    (cus-test-message cus-test-errors)
+    cus-test-errors))
 
 (defun cus-test-deps ()
   "Run a verbose version of `custom-load-symbol' on all atoms.

test/lisp/custom-tests.el

@@ -147,4 +147,9 @@
                                (widget-apply field :value-to-internal origvalue)
                                "bar"))))))
 
+(ert-deftest check-for-wrong-custom-types ()
+  :tags '(:expensive-test)
+  (load (concat installation-directory "admin/cus-test.el"))
+  (should (null (cus-test-opts t))))
+
 ;;; custom-tests.el ends here