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/user_search_spec.rb

@@ -37,9 +37,9 @@ RSpec.describe UserSearch do
   end
 
   context "with a secure category" do
-    fab!(:user) { Fabricate(:user) }
+    fab!(:user)
     fab!(:searching_user) { Fabricate(:user) }
-    fab!(:group) { Fabricate(:group) }
+    fab!(:group)
     fab!(:category) { Fabricate(:category, read_restricted: true, user: user) }
 
     before_all do