datastore: cleanup integration tests
datastore: cleanup integration tests

All integration tests renamed TestIntegration_* as with other
integration tests. Also added subtests to one test.

Change-Id: Ie832a086e60bbcbb018f6603f028dcb1353f54a8
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/40992
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Tyler Bui-Palsulich <b41e4aa57d2ad7eb1b29835a1b483e209dcc3c0e@google.com>
Reviewed-by: Emmanuel Odeke <bd234ba4276433f0e5fc7a8fa2d18274fa711567@orijtech.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go