Find test helper

Summary:
Since some ECM change, all binaries end up in $builddir/bin
This doesn't keep relative paths, so we need to adjust a test to find
the helper in the same directory.

Test Plan: Test passes

Reviewers: bshah

Reviewed By: bshah

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D8105