DEV: Allow fab! without block (#24314)

The most common thing that we do with fab! is:

    fab!(:thing) { Fabricate(:thing) }

This commit adds a shorthand for this which is just simply:

    fab!(:thing)

i.e. If you omit the block, then, by default, you'll get a `Fabricate`d object using the fabricator of the same name.

plugins/chat/spec/system/chat_summarization_spec.rb

@@ -2,7 +2,7 @@
 
 RSpec.describe "Summarize a channel since your last visit", type: :system, js: true do
   fab!(:current_user) { Fabricate(:user) }
-  fab!(:group) { Fabricate(:group) }
+  fab!(:group)
   let(:plugin) { Plugin::Instance.new }
 
   fab!(:channel) { Fabricate(:chat_channel) }