Attempt to fix Ubuntu 16.04 build, that is using Qt 5.5.

Summary:
Restricts fixture-based test to QT>5.9.
Removes initial condition test on kstars date for QT<5.8.
This is a no-brain workaround to return build on Ubuntu 16.04 to stable.

Test Plan: Local build OK, check Launchpad build.

Reviewers: mutlaqja

Reviewed By: mutlaqja

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D28117