test_config.rb: fix library path

* test/mkmf/test_config.rb (test_dir_config): fix expected library
  path.  [ruby-core:57535] [Bug #8972]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43106 b2dd03c8-39d4-4d8f-98ff-823fe69b080e