test_logdevice.rb: run iff timezene supported

* test/logger/test_logdevice.rb (test_shifting_dst_change): test
  only on platforms where timezone is supported.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45073 b2dd03c8-39d4-4d8f-98ff-823fe69b080e