Add a skylab_model field to target test requirements HwTest proto.

Add a skylab_model field to the target test requirements HwTest protocol
buffer. This will be used to make more specific test configurations for
release builders.

BUG=None
TEST=generate.sh

Change-Id: I5abebca3cc93949a728b2dccb69036a283a1362c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2298213
Reviewed-by: Sean Abraham <seanabraham@chromium.org>
Commit-Queue: David Burger <dburger@chromium.org>
Tested-by: David Burger <dburger@chromium.org>
2 files changed