Use lexical-binding in check-declare.el and add tests

* lisp/emacs-lisp/check-declare.el: Use lexical-binding.
(check-declare-warn): Silence byte compiler warning about unused
lexical argument.

* test/lisp/emacs-lisp/check-declare-tests.el: New file with tests
for check-declare.el.

1 files changed, 2 insertions, 2 deletions

diff --git a/lisp/emacs-lisp/check-declare.el b/lisp/emacs-lisp/check-declare.el
index 144385ea27c..52cda95f4c1 100644
--- a/lisp/emacs-lisp/check-declare.el
+++ b/lisp/emacs-lisp/check-declare.el
@@ -1,4 +1,4 @@
-;;; check-declare.el --- Check declare-function statements
+;;; check-declare.el --- Check declare-function statements  -*- lexical-binding: t; -*-
 
 ;; Copyright (C) 2007-2020 Free Software Foundation, Inc.
 
@@ -248,7 +248,7 @@ TYPE is a string giving the nature of the error.
 Optional LINE is the claim's line number; otherwise, search for the claim.
 Display warning in `check-declare-warning-buffer'."
   (let ((warning-prefix-function
-         (lambda (level entry)
+         (lambda (_level entry)
            (insert (format "%s:%d:" (file-relative-name file) (or line 0)))
            entry))
         (warning-fill-prefix "    "))