bigtable: break down TestIntegration_Client
bigtable: break down TestIntegration_Client

Splits TestIntegration_Client into a couple of
focused and smaller integration tests:
* TestIntegration_ArbitraryTimestamps
* TestIntegration_ConditionalMutations
* TestIntegration_DeleteRow
* TestIntegration_HighlyConcurrentReadsAndWrites
* TestIntegration_LargeReadsWritesAndScans
* TestIntegration_PartialReadRows
* TestIntegration_ReadModifyWrite
* TestIntegration_ReadRowList

Fixes #1421

Change-Id: I9ba19267f7ee990161292108c2ca6b38fd854a83
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/40952
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>
Reviewed-by: Tyler Bui-Palsulich <b41e4aa57d2ad7eb1b29835a1b483e209dcc3c0e@google.com>

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