solanum: fix MOTD

Previously this defaulted to the default MOTD in the solanum source
tree, and I don't want my friends to laugh at me. Includes a patch to
the tests to ensure that the MOTD is actually set.

This replicates the fix done in #109705 (solanum is a fork of charybdis,
so they share fundamental logic for this).

Signed-off-by: Christine Dodrill <me@christine.website>
(cherry picked from commit b1fe9fab6fcfa4cdefe7370ba264b281e48de0f8)

pkgs/servers/irc/solanum/default.nix

@@ -26,6 +26,10 @@ stdenv.mkDerivation rec {
     ./bandb.patch # https://github.com/solanum-ircd/solanum/issues/156
   ];
 
+  postPatch = ''
+    substituteInPlace include/defaults.h --replace 'ETCPATH "' '"/etc/solanum'
+  '';
+
   configureFlags = [
     "--enable-epoll"
     "--enable-ipv6"