fix: wait for profiling session shutdown

[vaind]

This was reproducible on Linux in our tests, although they did not fail:

error: There is no currently active test.
    Test: Sentry.Profiling.Tests.SamplingTransactionProfilerTests.EventPipeSession_ReceivesExpectedCLREvents
    Message: [Warning 00:00:01.05]: Error during sampler profiler EventPipeSession processing.
    Exception: System.ObjectDisposedException: Cannot access a disposed object.
Object name: 'Microsoft.Diagnostics.NETCore.Client.ExposedSocketNetworkStream'.
   at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 count)
   at FastSerialization.IOStreamStreamReader.Fill(Int32 minimum) in /home/ivan/dev/sentry-dotnet/modules/perfview/src/FastSerialization/StreamReaderWriter.cs:line 872
   at FastSerialization.MemoryStreamReader.ReadByte() in /home/ivan/dev/sentry-dotnet/modules/perfview/src/FastSerialization/StreamReaderWriter.cs:line 79
   at FastSerialization.Deserializer.ReadTag() in /home/ivan/dev/sentry-dotnet/modules/perfview/src/FastSerialization/FastSerialization.cs:line 2139
   at FastSerialization.Deserializer.FindEndTag(SerializationType type, IFastSerializable objectBeingDeserialized) in /home/ivan/dev/sentry-dotnet/modules/perfview/src/FastSerialization/FastSerialization.cs:line 2037
   at FastSerialization.Deserializer.ReadObjectDefinition(Tags tag, StreamLabel objectLabel) in /home/ivan/dev/sentry-dotnet/modules/perfview/src/FastSerialization/FastSerialization.cs:line 1964
   at FastSerialization.Deserializer.ReadObject() in /home/ivan/dev/sentry-dotnet/modules/perfview/src/FastSerialization/FastSerialization.cs:line 1411
   at Microsoft.Diagnostics.Tracing.EventPipeEventSource.Process() in /home/ivan/dev/sentry-dotnet/modules/perfview/src/TraceEvent/EventPipe/EventPipeEventSource.cs:line 160
   at Microsoft.Diagnostics.Tracing.Etlx.TraceLogEventSource.Process() in /home/ivan/dev/sentry-dotnet/modules/perfview/src/TraceEvent/TraceLog.cs:line 4519
   at System.Threading.Tasks.Task`1.InnerInvoke()
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location ---
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot, Thread threadPoolThread)

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