[NUnit] Improve support for guiunit

When running on guiunit there is no need to parse the resulting xml
file, since all results come through the tcp connection.

Improved TcpTestListener. It now properly reports the status for all
the sections (nodes) of a test suite.

Fixed 15165 - Xamarin Studio cannot run single tests using an external
test runner correctly.
[NUnit] Fix minor status reporting issue
[NUnit] There is no need to explicitly initialize the results pad
[NUnit] Improve reporting of Ignored status