PHPUnit classes don't exist in later versions of PHPUnit

[danielmorell]

Trying to run the PHPUnit test suite produces warnings when running the tests in PHPUnit 10 or above.

[15-Aug-2025 20:58:56 UTC] PHP Warning:  Class "PHPUnit\Framework\Error\Deprecated" not found in /wordpress-phpunit/includes/phpunit6/compat.php on line 8
[15-Aug-2025 20:58:56 UTC] PHP Warning:  Class "PHPUnit\Framework\Error\Notice" not found in /wordpress-phpunit/includes/phpunit6/compat.php on line 9
[15-Aug-2025 20:58:56 UTC] PHP Warning:  Class "PHPUnit\Framework\Error\Warning" not found in /wordpress-phpunit/includes/phpunit6/compat.php on line 10
[15-Aug-2025 20:58:56 UTC] PHP Warning:  Class "PHPUnit\Framework\Warning" not found in /wordpress-phpunit/includes/phpunit6/compat.php on line 12
[15-Aug-2025 20:58:56 UTC] PHP Warning:  Class "PHPUnit\Framework\TestListener" not found in /wordpress-phpunit/includes/phpunit6/compat.php on line 15

I think instead of blindly calling class_alias() on each of these, it would be better to wrap each in a call to class_exists(). This could easily be done in a foreach loop.

This PR resolves the warning triggered when running the PHPUnit test suite with PHPUnit 10 or higher. Instead of directly using class_alias, it first check whether the class exist or not. This ensures that the alias is only created when the class is available.

Trac ticket: https://core.trac.wordpress.org/ticket/63833

[SergeyBiryukov]

In 60693:

Build/Test Tools: Check if PHPUnit classes exist before calling class_alias().

This resolves Class not found warnings when running the tests in PHPUnit 10 or later.

Follow-up to [40536], [51570].

Props danielmorell, devasheeshkaul, SergeyBiryukov.
Fixes #63833.