Update dependency NUnit to v3.13.3 (#547)

+semver:patch

Author: renovate[bot]

src/FluentNHibernate.Testing/FluentNHibernate.Testing.csproj

@@ -22,7 +22,7 @@
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.3.2" />
     <PackageReference Include="FakeItEasy" Version="4.9.2" />
     <PackageReference Include="FluentAssertions" Version="4.19.4" />
-    <PackageReference Include="NUnit" Version="3.10.1" />
+    <PackageReference Include="NUnit" Version="3.13.3" />
     <PackageReference Include="NUnit3TestAdapter" Version="3.10.0" />
     <PackageReference Include="System.Data.SQLite.Core" Version="1.0.113.1" />
   </ItemGroup>