[TASK] Provide composer.json to fixture extensions

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: #100783
Releases: main, 12.4
Change-Id: I476864b528eaab1acec0a950c85d5ff92be7b516
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78942


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