Fix warnings during test runs

`warning: instance variable @Omitted not initialized`

test/drb/drbtest.rb

@@ -90,7 +90,7 @@ def setup_service(service_name)
  end
 
  def teardown
- return if @omitted
+ return if instance_variable_defined?(:@omitted) && @omitted
  @ext.stop_service if defined?(@ext) && @ext
  if defined?(@service_name) && @service_name
  @drb_service.manager.unregist(@service_name)