[bbagent]: add unit test for download cipd
[bbagent]: add unit test for download cipd

This change also adds a testBBClient that
saves the UpdateBuild requests sent to it in order make assertions about the contents.

Bug:1346754
Change-Id: I94fcf369a3e96c5456b54b7a388a223d0ff49a63
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3821925
Commit-Queue: Yulan Lin <fa22fbbce8ab65d96ec9925b9c61a0c0692eb649@google.com>
Reviewed-by: Chan Li <332b6c56634d0f63001046c664a2370006a02420@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go