filemetadatadatedisplaytest: Build and pass with < Qt5.10

Summary:
QFile::setFileTime was introduced in Qt5.10

Fixes build failures in CI of D11242

Test Plan:
#if QT_VERSION < QT_VERSION_CHECK(5, 99, 0)
make && make test

#if QT_VERSION < QT_VERSION_CHECK(5, 10, 0)
make && make test

Reviewers: ngraham, bruns, Baloo, Dolphin

Subscribers: mlaurent, lbeltrame

Tags: Baloo

Differential Revision: https://phabricator.kde.org/D12093