autopkgtest should take installed lib

--- a/test/setup.rb
+++ b/test/setup.rb
@@ -10,7 +10,7 @@
   Test::Unit::TestCase.test_order = :random
 end
 
-require File.expand_path("../../lib/logging", __FILE__)
+require "logging"
 
 module TestLogging
   module LoggingTestCase