Fix the building of tests in parallel

Summary:
tests depend on kded module, for this check was added in 34acf3 but
6cc50d ported plugin to json metadata, and in process renmaed target.

This fixes flaky build behavior on build.kde.org

Test Plan:
First cmake was throwing error about,

The dependency target "kded_kscreen" of target "testgenerator" does not exist.

rebuilt with change and now doesn't

Reviewers: sebas, Plasma

Reviewed By: sebas, Plasma

Subscribers: plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D1546