Resolve dependencies with phpunit version installed by simple-phpunit

[jderusse]

SimplePhpunit downloads the version 9.6 of PhpUnit, but some dependencies might requires another version of PHPUnit.
As a result, we have 2 differents version of the library installed in the vendors and the autoloader might load the classes in a different order.

Leading to errors like:

PHP Fatal error: Declaration of PHPUnit\Framework\TestSuite::run(): void must be compatible with PHPUnit\Framework\Test::run(?PHPUnit\Framework\TestResult $result = null): PHPUnit\Framework\TestResult in /home/runner/work/aws/aws/vendor/phpunit/phpunit/src/Framework/TestSuite.php on line 328