metrics: Fix metrics_uploader on VMs

metrics_uploader must be in testing mode so that it does not try to grab the
real hardware id (not available on VMs).

BUG=chromium:413256
TEST=FEATURES=test emerge-amd64-generic metrics.
TEST=platform_MetricsUploader succeeds on a gizmo VM.

Change-Id: I9e508c8661dfdb7933161b0d41ef4cf9bd7db2c6
Reviewed-on: https://chromium-review.googlesource.com/217760


Reviewed-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>