Provide a ProgressTraceListener to redirect Trace output to the NUnit Progress output

[maettu-this]

This issue is based on discussions at TestCentric/testcentric-gui#1043 and nunit/nunit3-vs-adapter#718 but more belongs to NUnit itself.

Similar to VS Test Adapter documentation which describes a workaround to activate Trace and Debug output, @CharliePoole has hinted "possible to create a listener which would capture the output and send it as a Progress event or write it to stderr, but I don't believe such a thing exists at this time" which I implemented at TestCentric/testcentric-gui#1043 (comment). It is simple and works.

Same as the ConsoleTraceListener class is provided by the .NET Framework, I propose the NUnit framework provides this NUnit specific ProgressTraceListener ready for use by the NUnit users. I don't ask NUnit to always have this listener active, it shall only provide the class and document its purpose and how to use it.