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Headless tests were scattered between /lib and /test directories.
Moving them to where they all belong helps to keep things nice and tidy.

Change-Id: I0aee0ae4da8e1f19dd45beb6cda2f108d9ec26a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2511751
Commit-Queue: Peter Kvitek <kvitekp@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824409}