TYPO3 Core

to improve the footprint, the tests are excluded by using the .gitattributes file.
Use --prefer-source and read about it at #77205

Thanks for the hint.

We have added "preferred-install": "source" to our composer.json config and all required Tests-folders are back now.

Nevertheless, I don't understand why the ZIP-sources on typo3.org are different from github.com

The zip sources on typo3.org (sourceforge to be precise) are ready-to-use sources, which do not require composer.
In contrast the git sources require composer to install the dependencies. This of course also applies for the zip files generated by github.