Make random test deterministic

[hugobuddel]

Fixes https://github.com/AstarVienna/ScopeSim/actions/runs/11993399220/job/33434342724

>       assert np.sum(fov_back.hdu.data) == approx(sum_orig, rel=1E-2)
E       assert 146.45611957005417 == 148.0 ± 1.5e+00
E         comparison failed
E         Obtained: 146.45611957005417
E         Expected: 148.0 ± 1.5e+00

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.04%. Comparing base (dde7c34) to head (603d61e).
Report is 61 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #512   +/-   ##
=======================================
  Coverage   77.04%   77.04%           
=======================================
  Files          66       66           
  Lines        8204     8204           
=======================================
  Hits         6321     6321           
  Misses       1883     1883           

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[teutoburg]

Thanks! I've been bothered by these random fails for a while, but never quite enough to do something about it. Let's just this was the only one of its kind...