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This simplifies some redundant logic in tests and allows future tests to
create multiple devices.

Bug: chromium:1059777

Change-Id: I1ae7d91090a5c13269513328691fab7fed4374a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2095121Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748291}