Add dummy log device and fix to use it
 * remove setting global logger in lib/fluent/test.rb
   * developer cannot understand when this assignment happen
   * global logger assignment exists in test/helper
 * dummy logger should contain messages that logger writes
   * to assert logger's output from core/plugin tests
 * test dummy logger is set in default in create_driver
   * to test logger message in configure
Merge pull request #308 from tagomoris/test_dummy_logger

Add dummy log device and fix to use it