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/models/topic_user_spec.rb

@@ -72,7 +72,7 @@ RSpec.describe TopicUser do
   it { is_expected.to belong_to :user }
   it { is_expected.to belong_to :topic }
 
-  fab!(:user) { Fabricate(:user) }
+  fab!(:user)
 
   let(:topic) do
     u = Fabricate(:user)