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/directory_items_controller_spec.rb

@@ -1,9 +1,9 @@
 # frozen_string_literal: true
 
 RSpec.describe DirectoryItemsController do
-  fab!(:user) { Fabricate(:user) }
-  fab!(:evil_trout) { Fabricate(:evil_trout) }
-  fab!(:walter_white) { Fabricate(:walter_white) }
+  fab!(:user)
+  fab!(:evil_trout)
+  fab!(:walter_white)
   fab!(:stage_user) { Fabricate(:staged, username: "stage_user") }
   fab!(:group) { Fabricate(:group, users: [evil_trout, stage_user]) }