[TASK] Provide composer.json to fixture extensions in ext:core

In preparation of composer based test setups, all fixture extensions
require fully functional `composer.json` files.

For starters, the minimal required `composer.json` is added. We will
adjust as we go. Extensions without classes don't receive PSR-4
class loading definitions as those are not needed.

As a convention, all fixture extensions have their composer name use the vendor `typo3tests`.

Classes will have the vendor `TYPO3Tests`.

This way, fixture extensions are easy to identify, both by providing
tests that use the extensions, as well as in development to not accidentally use such an extension or class.

Resolves: #100782
Releases: main, 12.4
Change-Id: If609f995dd31bd4549248436e660b3f86ce91687
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78914


Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>