Prevent DrupalWebTestCase from dropping the database

When you create a test class, that extends DrupalWebTestCase and you just override
setUp()
without calling:
parent::setUp();

then simpletest will delete your real database. This is not a real bug, since DrupalWebTestCase::setUp() is the place, where a test prefix is created, but it would be a real good idea to don't allow to delete the database tables, when there is no prefix.