Tests: Add unit tests for wp_admin_viewport_meta()

[pbearne]

This ticket adds comprehensive unit tests for the wp_admin_viewport_meta() function in wp-admin/includes/misc.php. The tests will cover the default meta tag generation, the admin_viewport_meta filter, and proper attribute escaping.

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Description:
This PR adds unit tests for the wp_admin_viewport_meta() function in wp-admin/includes/misc.php. These tests ensure that the function correctly outputs the viewport meta tag, handles the admin_viewport_meta filter, and properly escapes the content attribute.

The tests cover:

• Default viewport meta output.
• Custom viewport meta via the admin_viewport_meta filter.
• Ensuring no output is generated when the filtered value is empty.
• Security verification of attribute escaping in the output.

Trac ticket: https://core.trac.wordpress.org/ticket/65187

AI Disclosure:

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

[desrosj]

Adding the tests focus, which is used to indicate a ticket is solely focused on adding tests.

[SergeyBiryukov]

In 62366:

Tests: Add unit tests for wp_admin_viewport_meta().

Follow-up to [48412].

Props pbearne.
Fixes #65187.