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This change adds a ScopedVirtualU2fDevice to override normal discovery
in tests and always “discover” a VirtualU2fDevice. Using this, the
remaining appid tests are enabled.

Change-Id: I2836e1b9834b662032415fd0f45001cd5bcfae1d
Reviewed-on: https://chromium-review.googlesource.com/963097
Commit-Queue: Adam Langley <agl@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544162}