feat: capture code coverage in tests

[jbergstroem]

Warning

This is a work in progress. Do not merge.

Capture code coverage in CI and use codecov to track it.

Note: This only works on Linux due to missing functionality (ptrace/DWARF). Should be good enough for CI though.

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Todo

• Add Codecov (Example usage: https://github.com/jbergstroem/hadolint-gh-action)
• Investigate broken test
• Codespell

[jbergstroem]

@pgrange as-is, this requires us to set up codecov. Let me know if this is something that interests you. If not, we could just do a github comment or something.