[WriteTransactionTest] Clear mixup of QString and QByteArray

Summary:
touchFile expects a QString argument, not QByteArray. Use
QFile::encodeFileName() instead of QString::toUtf8().

Test Plan: ctest

Reviewers: Baloo, ngraham, astippich, poboiko

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D21844