Update Verify.Xunit to version 15.2.1

Author: 0xced

tests/Serilog.Formatting.Log4Net.Tests.csproj

@@ -15,7 +15,7 @@
     <PackageReference Include="Serilog" Version="2.10.0" />
     <PackageReference Include="Serilog.Enrichers.Environment" Version="2.2.0" />
     <PackageReference Include="Serilog.Enrichers.Thread" Version="3.1.0" />
-    <PackageReference Include="Verify.Xunit" Version="14.13.1" />
+    <PackageReference Include="Verify.Xunit" Version="15.2.1" />
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.3" />
   </ItemGroup>