test: probe whether the PHPUnit job detects a truncated suite

[Jozsef Damokos (jozsefdamokos)]

Probe for shopware/shopware#18667 — do not merge.

1. Why is this change necessary?

shopware/shopware#18667 asks us to give the after-sales plugin PHPUnit jobs the same "the suite really ran" safety net that the platform got in shopware/shopware#18675. Before choosing between the two options in the issue (reuse .github/actions/phpunit-run/action.yaml, or reimplement the check here), we need evidence about what this repository's PHPUnit job actually detects today.

2. What does this change do, exactly?

Adds a single test, tests/CompletionGuardDemoKillerTest.php, whose exit(0) terminates the PHPUnit process with a success exit code — the shopware/shopware#18560 incident class. phpunit.xml.dist runs the suite in executionOrder="random", so the process dies after an arbitrary prefix of the 234 test classes.

The Run PHPUnit step of .github/workflows/action-phpunit.yml only inspects the exit code, so the outcome of the PHPUnit job of the Integration workflow answers the question directly:

• green — the job cannot distinguish a full run from a truncated one, the issue is present here and needs a fix in this repository.
• red — platform's Shopware\Core\Test\PHPUnit\CompletionGuard, registered from TestBootstrapper::bootstrap() which tests/TestBootstrap.php calls, already covers this repository through the trunk platform this job installs. The failure message is then "PHPUnit terminated before the test runner finished the suite".

3. Describe each step to reproduce the issue or behaviour.

1. Open this PR and let the Integration / PHPUnit job run.
2. Compare the job result and the tail of the Run PHPUnit step output against the two cases above.

5. Checklist

• I have created the PR in draft status and only open it when it's ready for review
• This change has code comments where appropriate, especially for non-obvious lines of code