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/wizard/step_updater_spec.rb

@@ -123,7 +123,7 @@ RSpec.describe Wizard::StepUpdater do
       end
 
       context "with an existing default theme" do
-        fab!(:theme) { Fabricate(:theme) }
+        fab!(:theme)
 
         before { theme.set_default! }