[tests] Adds way to get backend device from WireHelper for tests.

- This allows for tests to get the backend device directly from the
  WireHelper which already provides the needed capabilities instead
  of needing to cache the backend device as a part of the injected
  proc implementations. This also allows tests to check toggles on
  multiple devices instead of only being able to check toggles on
  the most recently created device.

Change-Id: Ib77fdf82799c62be1cb32c0cfb13039042674468
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/304457
Commit-Queue: Brandon Jones <bajones@chromium.org>
Commit-Queue: Loko Kung <lokokung@google.com>
Auto-Submit: Loko Kung <lokokung@google.com>
Reviewed-by: Brandon Jones <bajones@chromium.org>