[Spanner]: Some tests are flaky

[amanda-tarafa]

When run conccurrently: https://source.cloud.google.com/results/invocations/23568d2c-60a0-4adc-8a51-cd3d7ec0f493/log

[amanda-tarafa]

https://source.cloud.google.com/results/invocations/63f4d186-c056-4fdb-af71-f10ba8df9b57/log

[amanda-tarafa]

googleapis/google-cloud-dotnet#8363 Should fix this.

[amanda-tarafa]

Failed CopyBackupTest.CopyBackup [51 ms]
  Error Message:
   Grpc.Core.RpcException : Status(StatusCode="FailedPrecondition", Detail="Cannot copy source backup projects/dotnet-docs-samples-tests/instances/my-instance/backups/my-test-database-backup because the backup is still being created. Please retry the operation once the pending backup is complete.")
  Stack Trace:
     at Grpc.Net.Client.Internal.HttpClientCallInvoker.BlockingUnaryCall[TRequest,TResponse](Method`2 method, String host, CallOptions options, TRequest request)
   at Grpc.Core.Interceptors.InterceptingCallInvoker.<BlockingUnaryCall>b__3_0[TRequest,TResponse](TRequest req, ClientInterceptorContext`2 ctx)
   at Grpc.Core.ClientBase.ClientBaseConfiguration.ClientBaseConfigurationInterceptor.BlockingUnaryCall[TRequest,TResponse](TRequest request, ClientInterceptorContext`2 context, BlockingUnaryCallContinuation`2 continuation)
   at Grpc.Core.Interceptors.InterceptingCallInvoker.BlockingUnaryCall[TRequest,TResponse](Method`2 method, String host, CallOptions options, TRequest request)
   at Google.Cloud.Spanner.Admin.Database.V1.DatabaseAdmin.DatabaseAdminClient.CopyBackup(CopyBackupRequest request, CallOptions options)
   at Google.Api.Gax.Grpc.ApiCall.GrpcCallAdapter`2.CallSync(TRequest request, CallSettings callSettings)
   at Google.Api.Gax.Grpc.ApiCallRetryExtensions.<>c__DisplayClass1_0`2.<WithRetry>b__0(TRequest request, CallSettings callSettings)
   at Google.Api.Gax.Grpc.ApiCall`2.<>c__DisplayClass12_0.<WithCallSettingsOverlay>b__1(TRequest req, CallSettings cs)
   at Google.Api.Gax.Grpc.ApiCall`2.Sync(TRequest request, CallSettings perCallCallSettings)
   at Google.Cloud.Spanner.Admin.Database.V1.DatabaseAdminClientImpl.CopyBackup(CopyBackupRequest request, CallSettings callSettings)
   at CopyBackupSample.CopyBackup(String sourceInstanceId, String sourceProjectId, String sourceBackupId, String targetInstanceId, String targetProjectId, String targetBackupId, DateTimeOffset expireTime) in C:\tmpfs\src\github\dotnet-docs-samples\spanner\api\Spanner.Samples\CopyBackup.cs:line 39
   at CopyBackupTest.CopyBackup() in C:\tmpfs\src\github\dotnet-docs-samples\spanner\api\Spanner.Samples.Tests\CopyBackupTest.cs:line 41

and

Failed CreateBackupTest.TestCreateBackup [174 ms]
  Error Message:
   Assert.Equal() Failure
Expected: AlreadyExists
Actual:   FailedPrecondition
  Stack Trace:
     at CreateBackupTest.TestCreateBackup() in C:\tmpfs\src\github\dotnet-docs-samples\spanner\api\Spanner.Samples.Tests\CreateBackupTest.cs:line 37

https://source.cloud.google.com/results/invocations/861571ba-2084-44cd-87fc-dbd92c9eb030/log