Add System.IO.FileSystem reference to Direct2D1.RenderTests project.

tests/Avalonia.RenderTests/Avalonia.Direct2D1.RenderTests.csproj

@@ -78,6 +78,11 @@
   <ItemGroup>
     <Service Include="{82A7F48D-3B50-4B1E-B82E-3ADA8210C358}" />
   </ItemGroup>
+  <ItemGroup>
+    <PackageReference Include="System.IO.FileSystem">
+      <Version>4.3.0</Version>
+    </PackageReference>
+  </ItemGroup>
   <Import Project="Avalonia.RenderTests.projitems" Label="Shared" />
   <Choose>
     <When Condition="'$(VisualStudioVersion)' == '10.0' And '$(IsCodedUITest)' == 'True'">