Add testing/{gtest,gmock}/dummy to facilitate rolling googletest.

googletest's directory structure has completely changed, and trybots
have trouble with the roll CL https://crrev.com/c/639953 because of
https://crbug.com/823586.

This CL works aroud the bug above, using the same method as
https://pdfium-review.googlesource.com/c/pdfium/+/29011 which helped
land https://pdfium-review.googlesource.com/c/pdfium/+/28791.

Bug: chromium:823586
Change-Id: Iffce1277c6b73b5cea96465a6035380cba0d103c
Reviewed-on: https://chromium-review.googlesource.com/974917Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52161}

testing/dummy

+dummy file

testing/gmock/dummy

+dummy file

testing/gtest/dummy

+dummy file