Buffer test output in headless a bit.

Just to newlines.  Cuts from profile by like 70%.

Seems like the bottleneck is Python, though...
Remove hack to skip messages from native.

Native is not so talkative anymore.
Add an internal comparison algorithm to headless.

This is slightly smarter than the test.py version, and should be
completely compatible.  It doesn't handle timeouts though.
Avoid writing to stdout/stderr from ffmpeg.

This will log some errors in the log even in release.
Support a timeout directly in headless.

Obviously, though, if headless infinite loops we're screwed.
Tweak ffmpeg logging in mp3/atrac as well.
Merge pull request #3804 from unknownbrackets/headless

Implement --compare in headless