Currently, running composer phpunit -- --filter "LanguageIntegrationTest::testParseFormattedNumber" using PHP 8.5 (on mediawiki/core, 3b065a367e9e) results in the following error:
1) MediaWiki\Tests\Language\LanguageIntegrationTest::testParseFormattedNumber with data set #11 ('en', NAN)
unexpected NAN value was coerced to string
/[path]/tests/phpunit/includes/Language/LanguageIntegrationTest.php:2142
=== Logs generated by test case
[...]
===The line of code in question (tests/phpunit/includes/Language/LanguageIntegrationTest.php:2142) is:
$this->assertEquals( $number, $normalisedNum );
From discussion at https://gerrit.wikimedia.org/r/c/mediawiki/core/+/1228117/comment/e8a0151d_c2549986/, it appears that this may be a PHP 8.5 compatibility issue with PHPUnit itself, rather than with MediaWiki code/tests. (Thanks for investigating it, @Ammarpad!!)
This task exists to track this issue downstream.
Upstream link: https://github.com/sebastianbergmann/phpunit/issues/6479