Lazily evaluate FileUpdateCheckerSharedTests when being included

To avoid "uninitialized constant ActiveSupport::Testing (NameError)"

activesupport/test/file_update_checker_shared_tests.rb

@@ -3,6 +3,8 @@
 require "fileutils"
 
 module FileUpdateCheckerSharedTests
+  def self.included(kls)
+    kls.class_eval do
       extend ActiveSupport::Testing::Declarative
       include FileUtils
 
@@ -293,4 +295,6 @@ def run(*args)
           new_checker([])
         end
       end
+    end
+  end
 end