Bump xunit from 2.8.1 to 2.9.0 (#130)

Bumps xunit from 2.8.1 to 2.9.0.


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Directory.Packages.props

@@ -8,7 +8,7 @@
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.10.0" />
     <PackageVersion Include="Microsoft.SourceLink.GitHub" Version="8.0.0" />
     <PackageVersion Include="Moq" Version="4.20.70" />
-    <PackageVersion Include="xunit" Version="2.8.1" />
+    <PackageVersion Include="xunit" Version="2.9.0" />
     <PackageVersion Include="xunit.runner.visualstudio" Version="2.8.1" />
   </ItemGroup>
 </Project>