[tumble] Make tests compatible with revised luci/gae APIs.
[tumble] Make tests compatible with revised luci/gae APIs.

R=tandrii@chromium.org, 50b2a565e2e78c292794832469a30ce5abc9959c@google.com

Bug: 1074114
Change-Id: I2fecc722c816fe60d617ed31162c779bba0af415
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2171635
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>

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