fix: discover tests in ErsatzTV.Tests

ErsatzTV.Tests references NUnit, NUnit3TestAdapter and Shouldly, but not
Microsoft.NET.Test.Sdk. Without it `dotnet test` builds the project and
then discovers nothing in it, so the two MultiSelectBase tests added
alongside the project in #2738 have never actually run.

Adding the package lets them execute; both pass, and a solution-wide
`dotnet test` still reports no failures.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

ErsatzTV.Tests/ErsatzTV.Tests.csproj

@@ -12,6 +12,7 @@
     </ItemGroup>
 
     <ItemGroup>
+      <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.8.1" />
       <PackageReference Include="NUnit" Version="4.6.1" />
       <PackageReference Include="NUnit.Analyzers" Version="4.14.0">
         <PrivateAssets>all</PrivateAssets>