Upgrade xunit to 2.4.1, add RetryHelper, add Xunit.SkippableFact

Author: chasedog

RedditSharpTests/RedditSharpTests.csproj

@@ -20,10 +20,12 @@
     <PackageReference Include="Microsoft.Extensions.Logging.Console" Version="2.2.0" />
     <PackageReference Include="Microsoft.DotNet.InternalAbstractions" Version="1.0.0" />
     <PackageReference Include="Microsoft.CodeCoverage" Version="16.2.0" />
-    <PackageReference Include="xunit" Version="2.3.1" />
+    <PackageReference Include="RetryHelper" Version="2.1.0" />
+    <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.3.1" />
     <PackageReference Include="Microsoft.Extensions.Configuration.UserSecrets" Version="2.2.0" />
     <PackageReference Include="Moq" Version="4.8.1" />
+    <PackageReference Include="Xunit.SkippableFact" Version="1.3.12" />
   </ItemGroup>
 
   <ItemGroup>