Original file naming caused conflicts after compiling the testproject when it had a reference to another (Xamarin) UWP project. Both projects had the default App.xaml and Mainpage.xaml as well as PNGs in Assets. *.xaml are compiled to *.xbf and, at some point are, copied to the bin/**/Debug folder, folder causing conflicts for the resources of the two apps, similar for the PNGs in /Assets (although the App under test should solely reside in the AppX folder).
UnitTestApp.xaml seems to be the only acceptable alternative name for App.xaml. Any other name causes an error stating that the 'Application main file must be named App.xaml'. MSTest projects use UnitTestApp.xaml.
Chris Maddock
Rob Prouse
Christoph Murczek