There are two options for testing out this patch:
1) Use an existing knsrc file which uses kpackage installation (such as plasma themes), which will use the fallback
2) Manually convert such a knsrc file, by removing the uninstall and installation commands from the knsrc file, and adding in an "Uncompress=kpackage" line instead
Both these should result in the KPackage path being used. You should see this on the command line when attempting to install an item, resulting in lines like "Using KPackage for installation", as well as more pleasant error reporting in the UI in the cases where something goes wrong.
To turn on debug output for KNewStuffCore, add QT_LOGGING_RULES="org.kde.knewstuff*=true" to your command line. For example, you can launch the test dialogue directly by launching the following from your build directory:
QT_LOGGING_RULES="org.kde.attica=true;org.kde.knewstuff*=true;log_attica_plugin=true" ./bin/khotnewstuff-dialog plasma-themes.knsrc