unittest: fix unittest under multi-DUT supported

Change function protocol, and add necessary mock function.

BUG=chromium:515749
TEST=run unittest
Change-Id: Ifef1041310241dfe32ab4e6b8b9acb6f6807b511
Reviewed-on: https://chromium-review.googlesource.com/293393
Reviewed-by: Wei-Ning Huang <wnhuang@chromium.org>
Tested-by: Claire Chang <tientzu@google.com>
Commit-Queue: Claire Chang <tientzu@google.com>
5 files changed