Stop assert dialogs popping up in netfx tests.

tests/Avalonia.Markup.Xaml.UnitTests/App.config

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <system.diagnostics>
+    <assert assertuienabled="false"/>
+  </system.diagnostics>
+</configuration>