chore(deps): update dependency xunit to v2.6.6 (#302)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

src/ReactiveMarbles.PropertyChanged.Tests/ReactiveMarbles.PropertyChanged.Tests.csproj

@@ -16,7 +16,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
-    <PackageReference Include="xunit" Version="2.6.4" />
+    <PackageReference Include="xunit" Version="2.6.6" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.5.6" />
     <PackageReference Include="Xunit.StaFact" Version="1.1.11" />
     <PackageReference Include="PublicApiGenerator" Version="11.1.0" />