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/onebox_controller_spec.rb

@@ -187,7 +187,7 @@ RSpec.describe OneboxController do
     end
 
     context "with local categories" do
-      fab!(:category) { Fabricate(:category) }
+      fab!(:category)
 
       it "oneboxes a public category" do
         get "/onebox.json", params: { url: category.url }