Issue #4686: Add ProgressTraceListener

[maettu-this]

As proposed in #4686.
Also fix to #4134.
Documentation update with separate docs PR.

[maettu-this]

I guess I need some support here. The Windows and MacOS tests indicate "1 tests failed" but searching the log for neither "fail" nor "Trace" nor "Progress" sheds any light into the failure.

On Linux, one of the newly added tests indeed fails:

1) Failed : NUnit.Framework.Tests.Diagnostics.ProgressTraceListenerTests.TestDebugIsDirectedToOutput 
Assert.That(TestResultListener.Outputs, Has.Count.EqualTo(1)) 
Expected: property Count equal to 1 
But was: 0 
at NUnit.Framework.Tests.Diagnostics.ProgressTraceListenerTests.TestDebugIsDirectedToOutput() in /_/src/NUnitFramework/tests/Diagnostics/ProgressTraceListenerTests.cs:line 108 

But the question is, why does this behave differently on Linux?

[maettu-this]

Review consideration regarding potential location of the listener:

• NUnit.Framework.Diagnostics.ProgressTraceListener (new namespace) pro: orthogonality with System.Diagnostics.ConsoleTraceListener
• NUnit.Framework.TestContext.ProgressTraceListener (class within TestContext) pro: in proximity with TestContext.Progress
• NUnit.Framework.ProgressTraceListener (top-level) pro: simplicity