[TYPO3-english] Unit testing in an existing installation - TYPO3 7.6.9

[Jennifer Koenig]

hi, I would like to run my unit tests in an existing installation of TYPO3 7.6.9. What I can find on the topic offers advice only for NEWLY installing the whole of TYPO3 from scratch:

https://wiki.typo3.org/Unit_Testing_TYPO3

I already have an existing installation of TYPO3 (and the latest version of composer) and only need to add unit-testing capabilities to it. How do I do this? Do I need to use the updated version of the phpunit extension, or is there another way?