Reworks the TestHttpServer threading to:

• Use an async method instead (so we can wait with a cancellation token)
• Use a cancellation token while awaiting the listener to return a request
  • GetContext() doesn't take a CTS directly, so we call Task.WaitAsync(CancellationToken ct) instead
• Adds some exception handling and discarding where necessary to ignore expected errors during shutdown

Closes coder/internal#603