Unique error code for each missing readme headers

[ernilambar]

For missing readme header, same error code missing_readme_header was used for all missing header items. Now error codes will be like:

• missing_readme_header_tested
• missing_readme_header_contributors

Unit test was missing for this error. Unit test has been added.

[github-actions]

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Co-authored-by: ernilambar <rabmalin@git.wordpress.org>
Co-authored-by: swissspidy <swissspidy@git.wordpress.org>
Co-authored-by: davidperezgar <davidperez@git.wordpress.org>

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