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/site_settings/validations_spec.rb

@@ -6,7 +6,7 @@ RSpec.describe SiteSettings::Validations do
   subject(:validations) { Class.new.include(described_class).new }
 
   describe "default_categories" do
-    fab!(:category) { Fabricate(:category) }
+    fab!(:category)
 
     it "supports valid categories" do
       expect {