Make sure we receive int exit code

[amureki]

• added a missing test case for an error
• fixed tests to not try to use non-existent files, which was leading to SystemExit(2) unrecognized arguments python
• made strict exit_code == 1 assertion

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (3d458d9) compared to base (1dfb345).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #45   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines          127       127           
=========================================
  Hits           127       127           

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