Tests: Add unit tests for _customizer_mobile_viewport_meta()

[pbearne]

This ticket adds unit tests for the _customizer_mobile_viewport_meta() function in wp-admin/includes/misc.php. The tests will verify that the function correctly appends the mobile-specific viewport settings to the existing viewport meta string.

Description:
This PR adds unit tests for the _customizer_mobile_viewport_meta() function in wp-admin/includes/misc.php. These tests ensure that the function correctly appends Customizer-specific viewport settings (minimum-scale=0.5,maximum-scale=1.2) to an existing viewport meta string, while handling trailing commas and empty input.

The tests cover:

• Appending to a standard viewport string.
• Handling of empty input strings.
• Correct trimming of single and multiple trailing commas before appending.

Trac ticket: https://core.trac.wordpress.org/ticket/65186

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 62352:

Tests: Add unit tests for _customizer_mobile_viewport_meta().

Follow-up to [48412].

Props pbearne.
Fixes #65186.