Replace exp_sig with expected_returncode

[fizyk]

No description provided.

[codecov-commenter]

Codecov Report

Merging #459 (99d71b2) into master (9ba60a1) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #459      +/-   ##
==========================================
- Coverage   83.98%   83.96%   -0.02%     
==========================================
  Files          22       22              
  Lines         999      998       -1     
==========================================
- Hits          839      838       -1     
  Misses        160      160              

Flag	Coverage Δ
linux	82.56% <100.00%> (-0.02%)	⬇️
macos	81.66% <100.00%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
src/mirakuru/base.py	97.61% <100.00%> (-0.02%)	⬇️
tests/executors/test_executor.py	100.00% <100.00%> (ø)

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