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/lib/upload_creator_spec.rb

@@ -3,7 +3,7 @@
 require "file_store/s3_store"
 
 RSpec.describe UploadCreator do
-  fab!(:user) { Fabricate(:user) }
+  fab!(:user)
 
   describe "#create_for" do
     describe "when upload is not an image" do