[NUnit] Don't assume that automatic updates always work

Only guiunit can give automatic updates, regardless of whether mdtool
is used or not. As such, even if we think automatic updates might
work we should protect against the case where they do not by loading
up the xml file at the end if automatic updates have not been received.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=15477
[NUnit] Use the pathname

The pathname is the correct thing to use, not the suitename
and testname.

The pathname handles tests subclassing other tests in the correct manner
[NUnit] GuiUnit as a project reference now works

I figured out a way to get the output filename for GuiUnit.exe
so i can now give that as the custom command when no other custom
command is supplied.

This means that both a direct binary reference on guiunit.exe and
also a project reference on it's csproj both result in your test
assembly being run using guiunit.exe