Fix missing error class and test for out_forward

[tagomoris]

This change is to use BufferedOutputTestDriver in test_out_forward.

I added some fixes in this change. These are related with each other.

• ForwardOutput plugin will raise an error for missing ACK, but it's not defined in the patch
  • It was missed because
• ObjectBufferedOutput does NOT have #format_stream and doing it in #emit directly
  • BufferedOutputTestDriver does NOT call #emit, and use #format_stream and chunk << data directly
• BufferedOutputTestDriver and TimeSlicedOutputTestDriver called block.call after chunking events
  • It's simply wrong, because d.emit in blocks don't work with such code
• Assigning Engine is already outdated