Hi, it depends.

Sometimes I use sqlite (specially with in memory option) If i need improve speed of test execution and I don't need to test some specific db functionalities (think about json fields or geo fields available on some specific database and not in other).

Sometimes i use mysql if i have mysql in production and i need to be sure to test with an environment similar to production.

But my advice is to test logic related with data trying to mock data as much as possible (in these case you don't need db at all)