Tests for the star detection

Summary:
This differential adds a few FITS load tests and benchmarks.
This alone is not enough to cover the whole detection algorithms, but is a beginning.

Test Plan: Build and run "testfitsviewer"

Reviewers: KStars, mutlaqja

Reviewed By: KStars, mutlaqja

Subscribers: murveit, kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D28635