Add tests for wp_staticize_emoji_for_email function

[pbearne]

A new unit test for the wp_staticize_emoji_for_email function has been added. This test provides verification for a variety of scenarios; from simple messages, messages with emojis, and adjustments based on the email's content type.

Trac ticket: https://core.trac.wordpress.org/ticket/60300

[github-actions]

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[swissspidy]

@pbearne this is just the same test as #5912 🤔

[github-actions]

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Props pbearne, swissspidy.

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