Add #[Group] attributes for Core 11.2 and PHPUnit 11

Problem/Motivation

In Core 11.2-dev 📌 Deprecate TestDiscovery test file scanning, use PHPUnit API instead Active was introduced, but broke Contrib testing and has been reverted.

Proposed resolution

Add #[Group('scheduler')] (or the appropriate group) to each phpunit test classes.

Remaining tasks