Run pixel tests via automated script.

On origin/master, we only have .ppm format support, so the
expected output files would take up a lot of space. Hence,
this may not get going until XFA hits with its .png
support.

Nor is there a good way to diff these; XFA provides this
for .png as well.

But this will provide at least one automated test to ensure
that we've got non-blank output, at least for one trivially
simple case.

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/926173002
3 files changed