A little fix to remove warning

[smpleader]

Steps to reproduce the issue

Use Joomla fw/ DI on my platform, install by composer

composer install "joomla/di"

Expected result

Install without warning

Actual result

There is a warning

Class Joomla\DI\Tests\StubInterface located in ./vendor/joomla/di/Tests/Stubs/stubs.php does not comply with psr-4 autoloading standard. Skipping.
Class Joomla\DI\Tests\Stub1 located in ./vendor/joomla/di/Tests/Stubs/stubs.php does not comply with psr-4 autoloading standard. Skipping.
Class Joomla\DI\Tests\Stub2 located in ./vendor/joomla/di/Tests/Stubs/stubs.php does not comply with psr-4 autoloading standard. Skipping.
Class Joomla\DI\Tests\Stub3 located in ./vendor/joomla/di/Tests/Stubs/stubs.php does not comply with psr-4 autoloading standard. Skipping.
Class Joomla\DI\Tests\Stub4 located in ./vendor/joomla/di/Tests/Stubs/stubs.php does not comply with psr-4 autoloading standard. Skipping.
Class Joomla\DI\Tests\Stub5 located in ./vendor/joomla/di/Tests/Stubs/stubs.php does not comply with psr-4 autoloading standard. Skipping.
Class Joomla\DI\Tests\Stub6 located in ./vendor/joomla/di/Tests/Stubs/stubs.php does not comply with psr-4 autoloading standard. Skipping.
Class Joomla\DI\Tests\Stub7 located in ./vendor/joomla/di/Tests/Stubs/stubs.php does not comply with psr-4 autoloading standard. Skipping.
Class Joomla\DI\Tests\Stub8 located in ./vendor/joomla/di/Tests/Stubs/stubs.php does not comply with psr-4 autoloading standard. Skipping.
Class Joomla\DI\Tests\Stub9 located in ./vendor/joomla/di/Tests/Stubs/stubs.php does not comply with psr-4 autoloading standard. Skipping.

System information (as much as possible)

Docker container Debain 10
Nginx with PHP8

Additional comments

This just need to separate file into many files. Is there any deeper reason we keep them into one file ?