When you test with TestFixture''''''s, on database-facing code, you need light fast database records.

http://www.zeroplayer.com/images/stuff/model_view_controller.png

Test cases use database records like the beads in the left panel (from ModelViewController); smaller beads are leaf records. As tests run, the records must flicker in and out. Some tests might test the database is naively empty, and so on.

Each test case...

* begin a transaction with the database
* execute one test case
* lower level commits get rolled back.
* on to next test case in microseconds

In all my decades coding have not yet seen that work correctly in auto test runners. Because...

''You can't change transaction isolation levels with a transaction open.''

[I don't think that's it.  I can't speak for other code, but the RelProject successfully uses extensive tests like the above -- nearly 500 at last count.]