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.

[maettu-this]

For completeness and those who would like to already use the resulting proposal, here's the code:

	/// <summary>
	/// Allows directing tracing or debugging output to <see cref="TestContext.Progress"/>.
	/// </summary>
	/// <remarks>
	/// Implemented following the <see cref="ConsoleTraceListener"/> implementation.
	/// </remarks>
	[HostProtection(Synchronization = true)]
	public class ProgressTraceListener : TextWriterTraceListener
	{
		/// <summary>
		/// Initializes a new instance of the <see cref="ProgressTraceListener"/> class with trace
		/// output written to <see cref="TestContext.Progress"/>.
		/// </summary>
		public ProgressTraceListener()
			: base(TestContext.Progress)
		{
		}
	}

To activate, place the following snippet into the one-time set-up method of either a test's fixture or the set-up fixture of a project:

System.Trace.Listeners.Add(new ProgressTraceListener());

Make sure to only add a listener once, e.g.:

if (!System.Trace.Listeners.OfType<ProgressTraceListener>().Any())
	System.Trace.Listeners.Add(new ProgressTraceListener());

Or again remove and dispose in the one-time tear-down, e.g.:

System.Trace.Listeners.Remove(_progressTraceListener);
_progressTraceListener.Dispose();