Update dependency NUnit to v4 (#629)

+semver:patch

Author: renovate[bot]

src/FluentNHibernate.Testing/FluentNHibernate.Testing.csproj

@@ -21,7 +21,7 @@
     <PackageReference Include="Microsoft.Data.Sqlite" Version="6.0.22" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.9.0" />
     <PackageReference Include="FakeItEasy" Version="8.1.0" />
-    <PackageReference Include="NUnit" Version="3.14.0" />
+    <PackageReference Include="NUnit" Version="4.1.0" />
     <PackageReference Include="NUnit.Analyzers" Version="4.0.1">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>