kcms/lookandfeel: make test metadata conform to requested format

Fixes QINFO  : KcmTest::initTestCase() KPackageStructure of KPluginMetaData(pluginId:"lookandfeel", fileName: "/var/tmp/gitlab_runner/.qttest/share/plasma/look-and-feel/lookandfeel/metadata.json") does not match requested format "Plasma/LookAndFeel"