[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