Emoji: Clean up the emoji browser support tests.

As new sets of emoji are added to Unicode, and browsers add support for them at random intervals, we'll inevitably need to add new tests to the emoji loader. This change makes it much easier to add new tests as they're needed.

Fixes #35300.


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git-svn-id: https://core.svn.wordpress.org/trunk@36783 1a063a9b-81f0-0310-95a4-ce76da25c4cd