Fix race condition in flaky test

[JamesNK]

Fixes:

  Failed AsyncUnaryCall_CancellationDuringBackoff_CanceledStatus [27 ms]
  Error Message:
     Expected: null
  But was:  System.Threading.CancellationTokenRegistration

  Stack Trace:
     at Grpc.Net.Client.Tests.Retry.HedgingCallTests.AsyncUnaryCall_CancellationDuringBackoff_CanceledStatus() in /home/runner/work/grpc-dotnet/grpc-dotnet/test/Grpc.Net.Client.Tests/Retry/HedgingCallTests.cs:line 336
   at NUnit.Framework.Internal.TaskAwaitAdapter.GenericAdapter`1.BlockUntilCompleted()
   at NUnit.Framework.Internal.MessagePumpStrategy.NoMessagePumpStrategy.WaitForCompletion(AwaitAdapter awaiter)
   at NUnit.Framework.Internal.AsyncToSyncAdapter.Await(Func`1 invoke)
   at NUnit.Framework.Internal.Commands.TestMethodCommand.RunTestMethod(TestExecutionContext context)
   at NUnit.Framework.Internal.Commands.TestMethodCommand.Execute(TestExecutionContext context)
   at NUnit.Framework.Internal.Execution.SimpleWorkItem.<>c__DisplayClass4_0.<PerformWork>b__0()
   at NUnit.Framework.Internal.ContextUtils.<>c__DisplayClass1_0`1.<DoIsolated>b__0(Object _)

[captainsafia]

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