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.

spec/requests/admin/plugins_controller_spec.rb

@@ -1,9 +1,9 @@
 # frozen_string_literal: true
 
 RSpec.describe Admin::PluginsController do
-  fab!(:admin) { Fabricate(:admin) }
-  fab!(:moderator) { Fabricate(:moderator) }
-  fab!(:user) { Fabricate(:user) }
+  fab!(:admin)
+  fab!(:moderator)
+  fab!(:user)
 
   describe "#index" do
     context "while logged in as an admin" do