Update test environment to fix tests on legacy PHP 7.2 with PHPUnit 8.5

[clue]

This changeset updates the test environment to fix tests on legacy PHP 7.2 with PHPUnit 8.5. This is needed to run the tests on legacy platforms due to recent upstream changes as discussed in clue/reactphp-redis#180.

Builds on top of clue/reactphp-redis#180 and #66 and others

[Copilot]

Pull request overview

This PR updates the test environment to support PHP 7.2 by adding PHPUnit 8.5 to the dependency constraints and simplifying the CI workflow configuration. Previously, PHP 7.2 fell into a gap where it couldn't use PHPUnit 9.6 (which requires PHP >= 7.3) and had to fall back to the older PHPUnit 5.7. This change fills that gap by adding PHPUnit 8.5, which is compatible with PHP 7.2-7.4.

Changes:

• Added PHPUnit 8.5 to the version constraint in composer.json to support PHP 7.2
• Simplified CI workflow by consolidating two conditional test execution steps into a single step with inline conditional logic

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
composer.json	Added ^8.5 to PHPUnit version constraints to enable PHPUnit 8.5 for PHP 7.2
.github/workflows/ci.yml	Simplified test execution by using inline conditional expression instead of separate conditional steps

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