Fix build warnings in the StreamDemuxerTests

[qmfrederik]

The StreamDemuxerTests were causing build warnings like this:

StreamDemuxerTests.cs(35,17): warning CS4014: Because this call is not awaited, execution of the current method continues before the call is completed. Consider applying the 'await' operator to the result of the call. 

This was cause by this line:

Task.Run(() => demuxer.Start());

which creates a background task but never awaits it.

StreamDemuxer.Start should return immediately, but it was possible for that call to block. I've fixed that.

I've also found an issue in the ReceiveAsync method of the MockWebSocket class, where it didn't honor the CancellationToken, and fixed that as well.

[brendandburns]

/lgtm
/approve

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This pull-request has been approved by: brendandburns, qmfrederik

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