Tests: Improve coverage for url_shorten() and test the previously unexercised $length parameter

[Soean]

This refactors the tests in tests/phpunit/tests/formatting/urlShorten.php and adds coverage that was missing.

Misplaced test argument

The case meant to test the optional $length parameter passed the length as the third argument to assertSame():

$this->assertSame( 'wordpress.org/about/philosophy/#…', url_shorten( 'http://wordpress.org/about/philosophy/#decisions' ), 31 );

That third argument is assertSame()'s $message, not the URL length. So url_shorten() ran with its default length of 35, the expected value happened to match the default output, and the assertion passed — while the $length parameter was never actually tested.

Changes

• Convert the tests to @dataProvider data sets with descriptive keys, so a failure points to the exact case.
• Separate the default-behaviour cases from the custom-length cases.
• Add real coverage for $length: the URL is kept when the length exceeds or equals the cleaned URL (pinning the strict > comparison) and shortened when it is smaller.
• Add a comment clarifying that a shortened URL is cut to ( $length - 3 ) characters before … is appended.

No production code is changed — this is a test-only improvement.

All tests pass locally via npm run test:php -- --filter Tests_Formatting_UrlShorten (9 tests, 9 assertions).

Trac ticket: https://core.trac.wordpress.org/ticket/64894

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