update xunit and mstest sdk

Author: inforithmics

src/ExCSS.Tests/ExCSS.Tests.csproj

@@ -17,9 +17,9 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.3.0" />
-    <PackageReference Include="xunit.runner.visualstudio" Version="2.2.0" />
-    <PackageReference Include="xunit" Version="2.2.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.6.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.5.0" />
+    <PackageReference Include="xunit" Version="2.5.0" />
   </ItemGroup>
 
   <ItemGroup>