fix(rome_cli): Fix max diagnostic counts

[MichaReiser]

This PR fixes the max diagnostics count for the linter.

The issue was that process_file reports a Success if a file has no diagnostic, even if the file has skipped diagnostics.

This PR changes process_file to return a Message if a file has reported diagnostics or any skipped diagnostics.

This PR further fixes an issue with the symlink tests that produced different output on unix/macos because the length of the resolved temp directory differs (/tmp vs /var/tmp.../), resulting in shorter or longer horizontal lines

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