Run menu-item :filter function before showing binding

Run menu-item :filter function before showing binding

* lisp/help.el (describe-map): Fix running `menu-item' :filter
functions.  This fixes a mistake in the previous conversion of this
defun from the old C function describe_map.  See the discussion in
Bug#39149.
* test/src/keymap-tests.el
(keymap---get-keyelt/runs-menu-item-filter)
(describe-buffer-bindings/menu-item-filter-show-binding)
(describe-buffer-bindings/menu-item-filter-hide-binding):
New tests.
(keymap-tests--test-menu-item-filter): New defun.