Buffered plugins for tests

Add buffered output plugins to test buffer plugins
 * buffered_stdout: show records in log console w/ buffering
 * buffered_null: show errors only brought by buffer plugins

These plugins are useful to develop 3rd party buffer plugins, so should be included in fluent/plugin/ directory.

And fix in_dummy not to stop to emit records beside any errors of buffer/output plugins to reproduce emit errors continuously.