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 #221 and others

[Copilot]

Pull request overview

This PR updates the test environment to support PHPUnit 8.5, which is needed for PHP 7.2 compatibility. The project currently supports PHP 7.1-8.5, and this change fills a gap in PHPUnit version support: PHPUnit 7.5 is used for PHP < 7.3, PHPUnit 8.5 for PHP 7.2-8.0, and PHPUnit 9.6 for PHP 7.3+. Additionally, the PR simplifies the CI workflow and improves Docker command reliability.

Changes:

• Add PHPUnit 8.5 to the version constraint in composer.json to support PHP 7.2 with a more modern PHPUnit version
• Simplify PHPUnit test execution in CI workflow by consolidating two conditional commands into one with inline conditionals
• Improve Docker workflow reliability by using named image tags instead of querying for the last built image

Reviewed changes

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

File	Description
composer.json	Adds PHPUnit 8.5 to the version constraint to bridge the gap between PHPUnit 7.5 and 9.6 for PHP 7.2 support
.github/workflows/ci.yml	Consolidates duplicate PHPUnit commands into a single command with conditional logic, and adds explicit Docker image tags for better reliability

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