DEV: switch message bus backend to memory for tests

This backend is a bit faster and well tested, this is part of a longer
term plan to have a `backend: :memory, threaded: false` type config for
message bus which we can use in test.

The threading in message bus causes all sorts of surprises in test, it will
be nice not to be beholden to them.

spec/requests/static_controller_spec.rb

@@ -13,6 +13,10 @@ describe StaticController do
       UploadCreator.new(file, filename).create_for(Discourse.system_user.id)
     end
 
+    before_all do
+      DistributedMemoizer.flush!
+    end
+
     after do
       DistributedMemoizer.flush!
     end