restored cli reference to dotnet-xunit and updated xUnit to beta packages so can run with the dotnet cli

Author: sergiomcalzada

tests/tests.csproj

@@ -8,9 +8,9 @@
     <PackageReference Include="Newtonsoft.Json" Version="10.0.2" />
     <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" />
-
+    <PackageReference Include="xunit" Version="2.3.0-beta3-build3705" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.3.0-beta3-build3705" />
+    <DotNetCliToolReference Include="dotnet-xunit" Version="2.3.0-beta3-build3705" />
   </ItemGroup>
   <ItemGroup>
     <None Include="assets/*" CopyToOutputDirectory="Always" />