Implement --compare in headless

Notes:
 * Not used yet by test.py.
 * This compare is very slightly smarter, and detects simple 1 inserted/omitted line drift.
 * The intention is for this to be faster (takes 16 seconds currently to run all tests for me.)
 * Kills some ffmpeg stdout/stderr logging in release, redirects to console.
 * Supports timeouts but won't detect hangs.
 * Should support the teamcity output just fine.
 * Might add support for running multiple tests in a row for even better perf, not yet supported.

-[Unknown]