Fix CI issues

[astrofrog]

This is just an experimental PR to try different things to fix the CI

[pkgw]

No dice, it looks like ...

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@ea2d404). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #196   +/-   ##
=========================================
  Coverage          ?   56.79%           
=========================================
  Files             ?       23           
  Lines             ?     1567           
  Branches          ?        0           
=========================================
  Hits              ?      890           
  Misses            ?      677           
  Partials          ?        0

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ea2d404...8efd486. Read the comment docs.

[astrofrog]

Ok so I haven't yet figured out a proper fix, but the summary of the issue is that somehow, after the loadFits call, the widget isn't quite reset to its original state, and changes to e.g. the background/foreground or the coordinates don't actually have any effect, so the images in test_full are just with the black background set in the image layer tests. For now I'll fix the CI by simply running the Image tests with an isolated Qt widget.