Provide option to output TRX results file

The `--xml` option that is provided is outputting xUnit's custom XML results file. To my knowledge, Visual Studio doesn't have support for loading this file into its Test Results window. Similarly for the Tests view in Azure DevOps pipelines. Without native support for these UI tools, this file isn't all that useful. We'd like to be able to run scenario-tests as part of the [VMR](https://github.com/dotnet/dotnet) build and have devs inspect the results in the Azure DevOps pipeline test view. To do that, there should be an option to output a Visual Studio results file format (TRX).