[datastore] enable safe get in tests.
[datastore] enable safe get in tests.

See bug for rationale & timeline.
To be landed on October 1st.

R=iannucci

Bug: 1242998
Change-Id: I23998d11e9b88147cad29647295d6ea29c5eb5e3
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3192861
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>

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