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[bot]]

Codecov Report

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

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

Additional details and impacted files

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