Correct GridDataInterpolator Testfile

[RuneDominik]

Found some shortcomings in this test file, nothing compromising the tests however.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (83ea9aa) 93.94% compared to head (41d2dd1) 93.95%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #235   +/-   ##
=======================================
  Coverage   93.94%   93.95%           
=======================================
  Files          54       54           
  Lines        2544     2548    +4     
=======================================
+ Hits         2390     2394    +4     
  Misses        154      154           

Impacted Files	Coverage Δ
pyirf/benchmarks/angular_resolution.py	100.00% <100.00%> (ø)
pyirf/benchmarks/tests/test_angular_resolution.py	100.00% <100.00%> (ø)
.../interpolation/tests/test_griddata_interpolator.py	100.00% <100.00%> (ø)
pyirf/spectral.py	77.31% <100.00%> (+0.78%)	⬆️

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