Fix Issue1225 regression test: use DefaultReferences instead of private References

The previous test commit used `References` which is a `private static` member
of ConverterTestBase and not accessible from the derived ByRefTests class.
Replace with `DefaultReferences.With()` (equivalent references) and also add
`ShowCompilationErrors = true` for consistency with other tests.

https://claude.ai/code/session_01AkwUvu3XuCdj3D4axoX4UX

Author: claude

Tests/CSharp/ExpressionTests/ByRefTests.cs

@@ -944,7 +944,7 @@ Public Sub Test(licenseName As String)
     End Sub
 End Class";
 
-        var options = new TextConversionOptions(References);
+        var options = new TextConversionOptions(DefaultReferences.With()) { ShowCompilationErrors = true };
         var languageConversion = new VBToCSConversion { ConversionOptions = options };
         var serviceTree = languageConversion.CreateTree(serviceFileContent);
         var callerTree = languageConversion.CreateTree(callerFileContent);