Add presubmit to avoid .in files in corpus

This CL adds a presubmit check so .in files are not added to corpus.

Change-Id: If99d93d4084fdbb871b5df5e788f4e28f244755b
Reviewed-on: https://pdfium-review.googlesource.com/6990
Reviewed-by: Lei Zhang <thestig@chromium.org>