AsyncTransactionManagerTest.asyncTransactionManagerWaitsUntilAsyncBatchUpdateHasFinished is flaky

[olavloite]

Error:  Failures: 
Error:    AsyncTransactionManagerTest.asyncTransactionManagerWaitsUntilAsyncBatchUpdateHasFinished:988 missing (1): class com.google.spanner.v1.CreateSessionRequest
---
expected   : [class com.google.spanner.v1.CreateSessionRequest, class com.google.spanner.v1.BatchCreateSessionsRequest, class com.google.spanner.v1.ExecuteBatchDmlRequest, class com.google.spanner.v1.CommitRequest]
but was    : [class com.google.spanner.v1.BatchCreateSessionsRequest, class com.google.spanner.v1.ExecuteBatchDmlRequest, class com.google.spanner.v1.CommitRequest]