[8.14](backport #39133) [Auditbeat/FIM/fsnotify]: remove time window where a child file operation of a directory can be lost

[mergify]

Proposed commit message

This PR fixes lost events for file integrity module of Auditbeat. Specifically as captured here there is tiny time window where if a child file get's created right after the internal dir contents snapshot of filepath.Walk and before we add the path to the fsnotify watcher, this file is now off the radar.

Mutliple of this tiny windows have been present during some buildkite CI runs (#39110 (comment), #39109 (comment), #39108 (comment))

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

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How to test this PR locally

cd auditbeat && for i in {1..15}; do mage build unitTest; done

Related issues

• Closes [Auditbeat/fim/fsnotify]: tiny time window that loses file events #39125

Use cases

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Screenshots

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Logs

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This is an automatic backport of pull request #39133 done by [Mergify](https://mergify.com).

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[mergify]

This pull request has not been merged yet. Could you please review and merge it @pkoutsovasilis? 🙏