Fix test of scriptedeffectsloader when run from build directory without install

Summary:
On build.kde.org all tests loading scripted effects are failing. This
seems to be because the effects are not installed.

To make this work, this change introduces the following changes:
 * scripted effects are copied to ${build}/bin, like all binaries
 * the test sets XDG_DATA_DIRS env variable to point to ${build}/bin

This change also needs to be added to further tests once this is
accepted. Furthermore it could be considered whether KPackageLoader
should consider the QCoreApplication::applicationDirPath in addition to
the GenericDataLocation. This would make KPackageLoader work much better
in a build tree only setup.

Test Plan: Test passes locally, obviously not tried on build.kde.org

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16682