Test stability - OpenTelemetry.Trace.Tests.BatchExportActivityProcessorTest.CheckShutdownExport

[reyang]

https://github.com/open-telemetry/opentelemetry-dotnet/actions/runs/8620692586/job/23628054054#step:6:6689

[xUnit.net 00:00:15.67]     OpenTelemetry.Trace.Tests.BatchExportActivityProcessorTest.CheckShutdownExport(timeout: 1) [FAIL]
[xUnit.net 00:00:15.67]       Assert.Single() Failure: The collection was empty
[xUnit.net 00:00:15.68]       Stack Trace:
[xUnit.net 00:00:15.69]         /home/runner/work/opentelemetry-dotnet/opentelemetry-dotnet/test/OpenTelemetry.Tests/Trace/BatchExportActivityProcessorTest.cs(145,0): at OpenTelemetry.Trace.Tests.BatchExportActivityProcessorTest.CheckShutdownExport(Int32 timeout)

[reyang]

It failed again https://github.com/open-telemetry/opentelemetry-dotnet/actions/runs/8807869536/job/24175888451

  Failed OpenTelemetry.Trace.Tests.BatchExportActivityProcessorTest.CheckShutdownExport(timeout: 1) [29 ms]
  Error Message:
   System.InvalidOperationException : Collection was modified; enumeration operation may not execute.
  Stack Trace:
     at System.Collections.Generic.List`1.Enumerator.MoveNext()
   at OpenTelemetry.Trace.Tests.BatchExportActivityProcessorTest.CheckShutdownExport(Int32 timeout) in /home/runner/work/opentelemetry-dotnet/opentelemetry-dotnet/test/OpenTelemetry.Tests/Trace/BatchExportActivityProcessorTest.cs:line 145

[reyang]

Also here https://github.com/open-telemetry/opentelemetry-dotnet/actions/runs/8729663667/job/23952065749.