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/poll/spec/lib/poll_spec.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 RSpec.describe DiscoursePoll::Poll do
-  fab!(:user) { Fabricate(:user) }
+  fab!(:user)
   fab!(:user_2) { Fabricate(:user) }
 
   fab!(:post_with_regular_poll) { Fabricate(:post, raw: <<~RAW) }