fixed package references:

* xUnit stable version
* xUnit.runner for VS
* missed testing package

Author: sergiomcalzada

tests/tests.csproj

@@ -1,18 +1,20 @@
 <Project Sdk="Microsoft.NET.Sdk">
-  <PropertyGroup>    
+  <PropertyGroup>
     <TargetFramework>netcoreapp1.1</TargetFramework>
     <IsPackable>false</IsPackable>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="xunit" Version="2.3.0-beta3-build3705" />
-    <DotNetCliToolReference Include="dotnet-xunit" Version="2.3.0-beta3-build3705" />
     <PackageReference Include="Microsoft.Rest.ClientRuntime" Version="3.0.3" />
     <PackageReference Include="Newtonsoft.Json" Version="10.0.2" />
-    <PackageReference Include="YamlDotNet.NetCore" Version="1.0.0" />    
+    <PackageReference Include="YamlDotNet.NetCore" Version="1.0.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.0.0" />
+    <PackageReference Include="xunit" Version="2.2.0" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.2.0" />
+
   </ItemGroup>
   <ItemGroup>
     <None Include="assets/*" CopyToOutputDirectory="Always" />
-  </ItemGroup>    
+  </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\src\KubernetesClient.csproj" />
   </ItemGroup>