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: #100773
Releases: main
Change-Id: Ibc1788031e39113dc1efda4ca7e4400e9cda444d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78886


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Jörg Bösche <typo3@joergboesche.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>