TYPO3 v9 has reached its end-of-life September 30th, 2021 and is not maintained by the community anymore. Looking for a stable version? Use the version switch on the top left.
You can order Extended Long Term Support (ELTS) here: TYPO3 ELTS.
In TYPO3, we're taking testing serious: When the core team releases a new TYPO3 version, they want to make sure it does not come with evil regressions (things that worked and stop working after update). This is especially true for patch level releases. There are various measures to ensure the system does not break: The patch review process is one, testing is another important part and there is more. With the high flexibility of the system it's hard to test "everything" manually, though. The TYPO3 core thus has a long history of automatic testing - some important steps are outlined in a dedicated chapter below.
With the continued improvements in this area an own testing framework has been established over the years that is not only used by the TYPO3 core, but can be used by extension developers or entire TYPO3 projects as well.
This chapter goes into details about automatic testing: Writing, maintaining and running them in various scopes. Have fun.