Created a method to diff the output dir against the golden copy
of the output dir. The goal with this is to simplify our end-to-end
test structure by removing the need to track the output files within
the test runner.
With all the golden output in one directory, this makes it easier to
setup end-to-end tests and even allows the golden files to be playable.
This should not affect the repo size too much as duplicate files will
only be stored in the git object store once.
Tests will migrate to this in separate CLs.
Change-Id: Id1be9f8f60c9e362b9b615445dd8ca7da996af4c
393f989839