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/pretty_text_spec.rb

@@ -3,8 +3,8 @@
 require "pretty_text"
 
 RSpec.describe PrettyText do
-  fab!(:user) { Fabricate(:user) }
-  fab!(:post) { Fabricate(:post) }
+  fab!(:user)
+  fab!(:post)
 
   before { SiteSetting.enable_markdown_typographer = false }
 
@@ -327,7 +327,7 @@ RSpec.describe PrettyText do
       let(:default_avatar) do
         "//test.localhost/uploads/default/avatars/42d/57c/46ce7ee487/{size}.png"
       end
-      fab!(:group) { Fabricate(:group) }
+      fab!(:group)
       fab!(:user) { Fabricate(:user, primary_group: group) }
 
       before { User.stubs(:default_template).returns(default_avatar) }