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/system/user_page/staff_info_spec.rb

@@ -1,8 +1,8 @@
 # frozen_string_literal: true
 
 describe "Viewing user staff info as an admin", type: :system do
-  fab!(:user) { Fabricate(:user) }
-  fab!(:admin) { Fabricate(:admin) }
+  fab!(:user)
+  fab!(:admin)
   let(:user_page) { PageObjects::Pages::User.new }
 
   before { sign_in(admin) }

spec/system/user_page/user_preferences_account_spec.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 describe "User preferences for Account", type: :system do
-  fab!(:user) { Fabricate(:user) }
+  fab!(:user)
   let(:user_account_preferences_page) { PageObjects::Pages::UserPreferencesAccount.new }
   let(:avatar_selector_modal) { PageObjects::Modals::AvatarSelector.new }
   before { sign_in(user) }

spec/system/user_page/user_preferences_interface_spec.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 describe "User preferences for Interface", type: :system do
-  fab!(:user) { Fabricate(:user) }
+  fab!(:user)
   let(:user_preferences_page) { PageObjects::Pages::UserPreferences.new }
   let(:user_preferences_interface_page) { PageObjects::Pages::UserPreferencesInterface.new }
 

spec/system/user_page/user_preferences_navigation_spec.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 describe "User page navigation menu", type: :system do
-  fab!(:user) { Fabricate(:user) }
+  fab!(:user)
   let(:everyone_group) { Group[:everyone] }
   let(:user_preferences_page) { PageObjects::Pages::UserPreferences.new }