Test comment-multi-line = nil auto fill case too

* test/lisp/progmodes/js-tests.el (js-mode-auto-fill): Test with
`comment-multi-line' both nil and non-nil.
* lisp/newcomment.el (comment-multi-line): Mark safe if it's a
boolean.
* etc/NEWS: Mention that `js-mode' now sets `comment-multi-line'.

etc/NEWS

@@ -624,6 +624,9 @@ initialization files.
 ---
 ** 'auto-revert-use-notify' is set back to t in 'global-auto-revert-mode'.
 
+---
+** JS mode now sets 'comment-multi-line' to t.
+
 ** CSS mode
 
 ---

lisp/newcomment.el

@@ -309,6 +309,7 @@ customize this variable.
 It also affects \\[indent-new-comment-line].  However, if you want this
 behavior for explicit filling, you might as well use \\[newline-and-indent]."
   :type 'boolean
+  :safe #'booleanp
   :group 'comment)
 
 (defcustom comment-empty-lines nil

test/lisp/progmodes/js-tests.el

@@ -89,16 +89,18 @@ if (!/[ (:,='\"]/.test(value)) {
 (ert-deftest js-mode-auto-fill ()
   (with-temp-buffer
     (js-mode)
-    (setq fill-column 70)
-    (insert "/* ")
-    (dotimes (_ 16)
-      (insert "test "))
-    (do-auto-fill)
-    ;; The bug is that, after auto-fill, the second line starts with
-    ;; "/*", whereas it should start with " * ".
-    (goto-char (point-min))
-    (forward-line)
-    (should (looking-at " \\* test"))))
+    (let ((fill-column 10)
+          (comment-multi-line t))
+      (insert "/* test test")
+      (do-auto-fill)
+      ;; Filling should continue the multi line comment.
+      (should (equal (buffer-string) "/* test\n * test"))
+      (erase-buffer)
+      (insert "/* test test")
+      (setq comment-multi-line nil)
+      (do-auto-fill)
+      ;; Filling should start a new comment on the next line.
+      (should (equal (buffer-string) "/* test */\n/* test")))))
 
 (ert-deftest js-mode-regexp-syntax-bug-25529 ()
   (dolist (regexp-contents '("[^[]"