test_logdevice.rb: no utime

* test/logger/test_logdevice.rb (test_shifting_midnight): do not
  set mtime to get rid of an error on Windows.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45099 b2dd03c8-39d4-4d8f-98ff-823fe69b080e