Add back NetFX.props framework path overriding.

tests/Avalonia.LeakTests/Avalonia.LeakTests.csproj

@@ -1,11 +1,12 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>net47</TargetFrameworks>
+    <TargetFrameworks>net461</TargetFrameworks>
   </PropertyGroup>
   <Import Project="..\..\build\JetBrains.dotMemoryUnit.props" />
   <Import Project="..\..\build\Moq.props" />
   <Import Project="..\..\build\Rx.props" />
   <Import Project="..\..\build\XUnit.props" />
+  <Import Project="..\..\build\NetFX.props" />
   <ItemGroup>
     <ProjectReference Include="..\..\src\Markup\Avalonia.Markup.Xaml\Avalonia.Markup.Xaml.csproj" />
     <ProjectReference Include="..\..\src\Markup\Avalonia.Markup\Avalonia.Markup.csproj" />

tests/Avalonia.UnitTests/Avalonia.UnitTests.csproj

@@ -35,7 +35,5 @@
   <Import Project="..\..\build\Moq.props" />
   <Import Project="..\..\build\Rx.props" />
   <Import Project="..\..\build\XUnit.props" />
-  <ItemGroup Condition="'$(TargetFramework)' == 'netcoreapp2.0'">
-      <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.0.0" />
-  </ItemGroup>
+  <Import Project="..\..\build\NetFX.props" />
 </Project>