Use unittest module instead of doctest.

Bug: 900161
Change-Id: I2e8265afb66345814c4c4b8e94618171b7aaaaf3
Reviewed-on: https://chromium-review.googlesource.com/c/1379441
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Shinya Kawanaka <shinyak@chromium.org>