chore: Add an autoload.php file for tests relying on core class

[come-nc]

Summary

Some applications are directly referring to OC::$loader in their tests bootstrap.php to be able to load Test\TestCase class.

This adds an autoload.php file that they can use instead, to allow core to refactor TestCase autoloading without breaking apps.

Pre-requisite for #52945

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[codecov]

❌ Unsupported file format

Upload processing failed due to unsupported file format. Please review the parser error message:

Error deserializing json

Caused by:
    expected value at line 1 column 1

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[come-nc]

/backport to stable31

[come-nc]

/backport to stable30

[come-nc]

/backport to stable29

[nickvergessen]

Should be documented in https://docs.nextcloud.com/server/latest/developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_32.html