Rename abstract class to prevent PHPUnit warning on 8.2

[andypost]

Faced it upgrading to 1.5.3 on Alpinelinux PHP 8.2.30, related to #24

ds

ds support => enabled
ds version => 2.0.0
PHPUnit 11.5.55 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.2.30
Configuration: /mnt/testing/php82-pecl-ds/src/ds-2.0.0/tests/phpunit.xml

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Time: 00:00.761, Memory: 16.00 MB

There was 1 PHPUnit test runner warning:

1) Class Ds\Tests\CollectionTest declared in /mnt/testing/php82-pecl-ds/src/ds-2.0.0/tests/vendor/php-ds/tests/tests/CollectionTest.php is abstract

OK, but there were issues!
Tests: 1891, Assertions: 5679, PHPUnit Warnings: 1.
>>> ERROR: php82-pecl-ds: check failed