Update test environment to make tests pass again

[clue]

Builds on top of #39, #22, reactphp/socket#305 and others
Supersedes parts of #41/#40

[Copilot]

Pull request overview

This PR updates the project’s test tooling and CI configuration so the PHPUnit test suite can run reliably again across supported runtimes (including legacy PHP and HHVM), aligning configuration with newer PHPUnit and workflow environments.

Changes:

• Update PHPUnit config schema references (legacy + modern) and align composer dev requirement to PHPUnit ^9.6.
• Add react/promise-timer as a dev dependency to support timer helpers used in tests.
• Refresh CI workflow environment (Ubuntu version, checkout action) and adjust HHVM execution to run via Docker.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
phpunit.xml.legacy	Generalizes legacy PHPUnit config comment wording.
phpunit.xml.dist	Updates modern PHPUnit schema reference to 9.6.
composer.json	Bumps PHPUnit dev constraint, adds react/promise-timer, and reformats autoload blocks.
.github/workflows/ci.yml	Moves CI to Ubuntu 24.04, adjusts PHP matrix, updates checkout action, and reworks HHVM steps to use Docker.

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