[client][cas] add tests for download stats from cas client.
[client][cas] add tests for download stats from cas client.

Bug: 1117004
Change-Id: I9eb487c2f3d3c000ac24dffd2e96d62d83bfdbd0
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2413871
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Auto-Submit: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py