ECMAddTests: set QT_PLUGIN_PATH so locally built plugins can be found

Summary:
This of course requires running the unittests via ctest rather than
launching the executable directly.

Test Plan:
ctest -V -R kurifiltertest in kio, which prints QCoreApplication::libraryPaths() at the beginning of the test.
With this commit, builddir/kio/bin is added at the front of the list, and the kurifilter plugins from kio's build are loaded.

Reviewers: cgiboudeaux, kfunk

Reviewed By: cgiboudeaux

Subscribers: Frameworks, Build System

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D8660