Fix running qtvideosink_autotest after requiring >= ECM 5.38

Summary:
Requiring at least ECM 5.38 comes with a different build dir layout to
help testing of not installed apps. But in combination with the old
add_test syntax it caused qtvideosink_autotest to fail because the test
executable wasn't found anymore. Using ecm_add_test fixes that.

Test Plan: Test was found after invoking "make test" and passes.

Reviewers: apol

Differential Revision: https://phabricator.kde.org/D27179