Unit test fixes

-- Fixes https://github.com/mozilla/mozjpeg/issues/132
-- Restores full bitwise compatibility with libjpeg-turbo whenever -revert is used
-- Adds -revert to the unit tests, so they actually work again (at least they can be used to regression test the underlying code)
-- Introduces a TJ_REVERT environment variable, which serves the same purpose with the TurboJPEG API as the -revert switch does with cjpeg