Add testing config for Release notes notification experiment.

Bug: b/156308432
Change-Id: I8634b4dcffe616f9ce950a6d9500e957f4e83867
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2400587


Commit-Queue: Rachel Carpenter <carpenterr@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805606}