Add tests for print_emoji_detection_script function

[pbearne]

A new unit test file is created to ensure that the print_emoji_detection_script function works correctly. The tests check whether the final output contains the expected string and ensure no output when printing the emoji detection script again.

Trac ticket: https://core.trac.wordpress.org/ticket/60301

[github-actions]

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[github-actions]

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Props pbearne.

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