Bring test coverage of src/dags to 100%

[hmgaudecker]

We were close already, but can't hurt to add a couple of tests.

[codecov]

Codecov Report

❌ Patch coverage is 98.14815% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.61%. Comparing base (1cfab77) to head (332b5dd).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
tests/test_signature.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #76      +/-   ##
==========================================
+ Coverage   95.32%   96.61%   +1.29%     
==========================================
  Files          26       26              
  Lines        1218     1272      +54     
==========================================
+ Hits         1161     1229      +68     
+ Misses         57       43      -14     

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