update to latest xunit

Rx.NET/Source/tests/Tests.System.Reactive/Tests.System.Reactive.csproj

@@ -16,8 +16,8 @@
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-    <PackageReference Include="xunit" Version="2.2.0" />
-    <PackageReference Include="xunit.runner.visualstudio" Version="2.2.0" />
+    <PackageReference Include="xunit" Version="2.3.0-beta1-build3561" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.3.0-beta1-build1279" />
     <ProjectReference Include="..\..\src\System.Reactive\System.Reactive.csproj" />
     <ProjectReference Include="..\..\src\System.Reactive.Observable.Aliases\System.Reactive.Observable.Aliases.csproj" />
     <ProjectReference Include="..\..\src\Microsoft.Reactive.Testing\Microsoft.Reactive.Testing.csproj" />