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