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/unsubscribe_key_spec.rb

@@ -17,7 +17,7 @@ RSpec.describe UnsubscribeKey do
   end
 
   describe "key" do
-    fab!(:user) { Fabricate(:user) }
+    fab!(:user)
     let!(:key) { UnsubscribeKey.create_key_for(user, UnsubscribeKey::DIGEST_TYPE) }
 
     it "has a temporary key" do