Don't run fitsviewer test if no fitsio
Summary: Should fix https://build.kde.org/view/Failing/job/Extragear/job/kstars/job/kf5-qt5%20WindowsMSVCQt5.14/ (tests are not working as fitsio not available on Windows)
Reviewers: KStars, TallFurryMan, mutlaqja
Reviewed By: KStars, TallFurryMan, mutlaqja
Subscribers: TallFurryMan, kde-edu
Differential Revision: https://phabricator.kde.org/D29388