Bump the dev-dependencies group in /tests with 2 updates

Bumps the dev-dependencies group in /tests with 2 updates: [Microsoft.NET.Test.Sdk](https://github.com/microsoft/vstest) and [xunit](https://github.com/xunit/xunit).


Updates `Microsoft.NET.Test.Sdk` from 17.7.2 to 17.8.0
- [Release notes](https://github.com/microsoft/vstest/releases)
- [Changelog](https://github.com/microsoft/vstest/blob/main/docs/releases.md)
- [Commits](microsoft/vstest@v17.7.2...v17.8.0)

Updates `xunit` from 2.5.1 to 2.6.6
- [Commits](xunit/xunit@2.5.1...2.6.6)

---
updated-dependencies:
- dependency-name: Microsoft.NET.Test.Sdk
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: dev-dependencies
- dependency-name: xunit
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: dev-dependencies
...

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

tests/OldIcelandicDictionaryTests.csproj

@@ -9,7 +9,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.7.2" />
-    <PackageReference Include="xunit" Version="2.5.1" />
+    <PackageReference Include="xunit" Version="2.6.6" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.5.6">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>