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This follows the convention for other directories, which is that tests live alongside the code they test.

Also update the readme.

Change-Id: I2f3ecd135825572f78930e09a99419ab5b8c257c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1900566
Auto-Submit: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712991}