Add ability to opt-in or out of transactions per-database in test
fixtures. Using the new `use_transactional_tests_for_database` method
you can override the `use_transactional_tests` default preference
for a particular database to opt-in (or out by passing `false`) to using
transactions. Also, using the new `skip_transactional_tests_for_database`
method we can easily opt-out of transactions for a database.

Co-authored-by: Morgan Mareve <mmareve@github.com>