Fix Gradient Check Test

[PaulJonasJost]

Closes #1495

Cause for this was running the gradient check in mode_res with a noise parameter. This always ran into a runtime error and was evaluated as False. Fixed with amici_model.setAddSigmaResiduals(True) and correct assertion. Not sure how this got in.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.35%. Comparing base (bc7d89a) to head (77ab5fa).
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Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1668      +/-   ##
===========================================
- Coverage    84.37%   84.35%   -0.02%     
===========================================
  Files          164      164              
  Lines        14320    14320              
===========================================
- Hits         12082    12080       -2     
- Misses        2238     2240       +2     

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[dweindl]

Not sure how this got in.

#690 (comment) :(

[dweindl]

The test/base/test_engine.py changes belong somewhere else, but the rest looks fine to me.