Summary:
This fixes some inconsistencies like
 - ecm_mark_as_test not called on all tests
 - ecm_mark_as_test called with the wrong prefix in the name
and it allows us to benefit from the automatic setting of
QT_PLUGIN_PATH, as done in ecm_add_test.

We could go further and use the LINK_LIBRARIES argument too, but
that doesn't really matter.

Test Plan: ctest

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Differential Revision: https://phabricator.kde.org/D21094