update to latest xunit

Ix.NET/Source/System.Interactive.Tests/System.Interactive.Tests.csproj

@@ -17,12 +17,12 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.3.0" />
-    <PackageReference Include="xunit.runner.visualstudio" Version="2.3.0-beta4-build3742 " />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.5.0" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.3.1" />
     <PackageReference Include="FluentAssertions" Version="4.19.2 " />
     
-    <PackageReference Include="xunit" Version="2.3.0-beta3-build3705" />
-    <DotNetCliToolReference Include="dotnet-xunit" Version="2.3.0-beta4-build3742 " />
+    <PackageReference Include="xunit" Version="2.3.1" />
+    <DotNetCliToolReference Include="dotnet-xunit" Version="2.3.1" />
 
   </ItemGroup>