Bump NodaTime.Testing from 3.1.6 to 3.1.8

Bumps [NodaTime.Testing](https://github.com/nodatime/nodatime) from 3.1.6 to 3.1.8.
- [Release notes](https://github.com/nodatime/nodatime/releases)
- [Changelog](https://github.com/nodatime/nodatime/blob/main/NodaTime%20Release.snk)
- [Commits](nodatime/nodatime@3.1.6...3.1.8)

---
updated-dependencies:
- dependency-name: NodaTime.Testing
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

samples/Directory.Packages.props

@@ -4,6 +4,6 @@
   </PropertyGroup>
   <ItemGroup>
     <PackageVersion Include="NodaTime" Version="3.1.6"/>
-    <PackageVersion Include="NodaTime.Testing" Version="3.1.6"/>
+    <PackageVersion Include="NodaTime.Testing" Version="3.1.8"/>
   </ItemGroup>
 </Project>