Honor BUILD_TESTING
Test Plan:
- Passed BUILD_TESTING=OFF option to cmake, no tests have been built;
- Didn't pass BUILD_TESTING option, all tests have been built.
Reviewers: KWin, graesslin
Reviewed By: KWin, graesslin
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D13163