Testing, specifically when having database dependant code

Swimming towards the deep end of the pool and now learning more about testing. 
In CI3 I used kenji's ci-phpunit-test with much success. Database testing has been made with a mysql server with a downloaded copy of the latest live database and making sure I only run non-destructive tests.


With CI4 I try to do the same, but find the following line from the $tests structure in Config\Database.php:67 a bit strange:

PHP Code:

'DBPrefix' => 'db_',  // Needed to ensure we're working correctly with prefixes live. DO NOT REMOVE. 


 
I have already nuked my DB twice by setting this to blank so I'm really missing something here ;-)