update to latest xunit

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

@@ -15,15 +15,12 @@
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-    <DotNetCliToolReference Include="dotnet-xunit" Version="2.3.0-beta5-build3745" />
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+    <PackageReference Include="xunit.runner.visualstudio" Version="2.3.1" />
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     <ProjectReference Include="..\..\src\System.Reactive\System.Reactive.csproj" />
     <ProjectReference Include="..\..\src\System.Reactive.Observable.Aliases\System.Reactive.Observable.Aliases.csproj" />
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