Refactor Emoji browser support tests to prevent repetition.

[peterwilsoncc]

The tests for Emoji browser support writes to and clears the canvas several times.

The code to do this is repeated a number of times and should be refactored. The repitition lead to a couple of (unreleased) bugs recently.

[peterwilsoncc]

In 41156.diff​ I've moved some repeated code into a new function.

[pento]

And it reduces the minified code by 166 bytes, what's not to love?

[peterwilsoncc]

In 41669:

Emoji: Refactor browser support tests.

Moves the repeated code in the emoji browser support test to a function to reduced the potential for errors and follow DRY principles.

Renames the "emoji4" test "emoji" as unicode version specific tests were removed in [38869].

Props peterwilsoncc, pento for code review.
Fixes #41156.