65186 misc customizer mobile viewport meta

[pbearne]

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

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