gnu: emacs-helpful: Fix build.

* gnu/packages/patches/emacs-helpful-fix-tests.patch: Squash a test fix for
Emacs 30.
* gnu/packages/patches/emacs-helpful-fix-signature.patch: New file.
* gnu/local.mk (dist_patch_DATA): Register it here.
* gnu/packages/emacs-xyz.scm (emacs-helpful)[patches]: Use it here.

gnu/packages/patches/emacs-helpful-fix-tests.patch

@@ -1,16 +1,18 @@
-From 233e254bbe44f70ca899a506b3706af489228fda Mon Sep 17 00:00:00 2001
-From: aagon <aymeric.agon@hotmail.fr>
-Date: Thu, 10 Aug 2023 20:03:52 +0200
-Subject: [PATCH] Repair helpful--display-implementations test for emacs 29.1
-
----
- test/helpful-unit-test.el | 8 ++++++--
- 1 file changed, 6 insertions(+), 2 deletions(-)
-
 diff --git a/test/helpful-unit-test.el b/test/helpful-unit-test.el
 index 0f5177b..0716b0d 100644
 --- a/test/helpful-unit-test.el
 +++ b/test/helpful-unit-test.el
+@@ -734,7 +734,9 @@ associated a lambda with a keybinding."
+   (should
+    (equal
+     (length (helpful--keymaps-containing #'helpful--dummy-command))
+-    2))
++    (if (< emacs-major-version 30)
++        2
++      3)))
+ 
+   ;; Undo keybinding.
+   (global-set-key (kbd "C-c M-S-c") nil)
 @@ -1089,8 +1089,12 @@ find the source code."
    (require 'xref)
    (helpful-function 'xref-location-marker)