logger.rb: fix r43511 for Windows

* lib/logger.rb (Logger::LogDevice::LogDeviceMutex#lock_shift_log):
  open file can't be removed or renamed on Windows.  [ruby-dev:47790]
  [Bug #9046]
* test/logger/test_logger.rb (TestLogDevice#run_children): don't use
  fork.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43513 b2dd03c8-39d4-4d8f-98ff-823fe69b080e