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/embeddable_host_spec.rb

@@ -152,7 +152,7 @@ RSpec.describe EmbeddableHost do
   end
 
   describe ".record_for_url" do
-    fab!(:embeddable_host) { Fabricate(:embeddable_host) }
+    fab!(:embeddable_host)
 
     it "returns the right record if given URL matches host" do
       expect(EmbeddableHost.record_for_url("https://#{embeddable_host.host}")).to eq(