Add a presubmit check to the test corpus.

The check makes sure PNGs have the right filename format.

BUG=pdfium:693

Change-Id: If489cf0e0bdff522aa7927620bacc2759356fafb
Reviewed-on: https://pdfium-review.googlesource.com/3512
Reviewed-by: Tom Sepez <tsepez@chromium.org>