Add unit tests for saveDomDocument() in wp-admin/includes/misc.php

[pbearne]

Description:
This PR adds unit tests for the saveDomDocument() function in wp-admin/includes/misc.php. These tests ensure that the function correctly saves a DOMDocument to a file and properly converts line endings to CRLF, which is required for IIS configuration files.

The tests cover:

• Successful saving of a DOMDocument to a specified path.
• Verification that line endings are converted to CRLF.

Trac ticket: https://core.trac.wordpress.org/ticket/65173

AI Disclosure:

• AI assistance: Yes
• Tool(s): Junie (JetBrains)
• Model(s): gemini-3-flash-preview
• Used for: Code analysis, test implementation, and workflow management.

[github-actions]

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[github-actions]

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