Update to latest xUnit VS Runner beta

This fixes the problem where the tests aren't runnable from the
CodeLens UI.

Author: bording

LibGit2Sharp.Tests/LibGit2Sharp.Tests.csproj

@@ -21,7 +21,7 @@
     <PackageReference Include="Moq" Version="4.7.8" />
     <PackageReference Include="xunit" Version="2.2.0" />
     <PackageReference Include="xunit.skippablefact" Version="1.3.1" />
-    <PackageReference Include="xunit.runner.visualstudio" Version="2.2.0" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.3.0-beta1-build1309" />
     <PackageReference Include="xunit.runner.console" Version="2.2.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.0.0" />
   </ItemGroup>