libweave: Move some of unittest_utils into public interface

User may want to make tests similar to those in libweave implementation.

BUG=brillo:1245
TEST='FEATURES=test emerge-gizmo buffet'

Change-Id: I18843645a59fa2323474157b95faf242d871fe2c
Reviewed-on: https://chromium-review.googlesource.com/289695
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Tested-by: Vitaly Buka <vitalybuka@chromium.org>