Add dotnet-xunit CLI package to Client.Tests project (required by CI build process).

Author: tintoy

test/Client.Tests/Client.Tests.csproj

@@ -18,8 +18,10 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.3.0-preview-20170628-02" />
-    <PackageReference Include="xunit" Version="2.2.0" />
-    <PackageReference Include="xunit.runner.visualstudio" Version="2.2.0" />
+    <PackageReference Include="xunit" Version="2.3.0" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.3.0" />
+
+    <DotNetCliToolReference Include="dotnet-xunit" Version="2.3.0" />
   </ItemGroup>
 
   <ItemGroup>