matrix-appservice-discord: increase test timeout

Hydra and my local machine are sometimes hitting the default timeout.
See https://hydra.nixos.org/build/138032455/nixlog/8/tail

pkgs/servers/matrix-appservice-discord/default.nix

@@ -43,7 +43,8 @@ in mkYarnPackage rec {
 
   doCheck = true;
   checkPhase = ''
-    yarn --offline test
+    # the default 2000ms timeout is sometimes too short on our busy builders
+    yarn --offline test --timeout 10000
   '';
 
   postInstall = ''