emacs: test upstream bug#81105 in the withPackages test

Currently, the test only passes on Emacs < 31.  We have to work around
it before upstream fixes the bug (if they decide to fix it).

https://debbugs.gnu.org/cgi/bugreport.cgi?bug=81105

pkgs/applications/editors/emacs/build-support/wrapper-test.nix

@@ -15,6 +15,10 @@ runCommand "test-emacs-withPackages-wrapper"
       ))
       git # needed by magit
     ];
+    env = {
+      # emulate a default NixOS env where INFOPATH is set like this (not ending with a ":")
+      INFOPATH = "/fake-info-dir1:/fake-info-dir2";
+    };
   }
   ''
     emacs --batch --eval="(require 'magit)"
@@ -23,5 +27,9 @@ runCommand "test-emacs-withPackages-wrapper"
     # transitive dependencies should be made available
     # https://github.com/NixOS/nixpkgs/issues/388829
     emacs --batch --eval="(require 'flx)"
+
+    # test that https://debbugs.gnu.org/cgi/bugreport.cgi?bug=81105 is fixed or worked around
+    emacs --batch --eval='(progn (package-activate-all) (info "(magit)Top"))'
+
     touch $out
   ''