fix: configure PHPUnit test runner

[chubes4]

Problem\n\ncomposer test invokes bare phpunit, but this repository has no tracked PHPUnit configuration or bootstrap file. PHPUnit therefore prints its usage and exits with status 2 instead of discovering the existing tests/*Test.php suite.\n\n## Reproduction\n\nsh\ncomposer test\n\n\n## Expected\n\nThe Composer test script should load the required WordPress test bootstrap/configuration and execute the tracked suite.\n\n## Actual\n\nPHPUnit prints usage and exits with code 2.\n\n## Scope\n\nDo not change versions or CHANGELOG.md; make the runner convention explicit and CI-usable.