Shouldn't the default be for a test to not use the DB at all? Like we discussed yesterday, we should disable the database entirely unless a test says it needs it.
The thing is, there's a lot of things which may use the i18n cache, etc without even intending too. It wouldn't hurt to make a clean DB on each test.
All caches (including i18n) should be disabled by bootstrap (unless we're doing a cache test?). Tests should be very narrowly focused, and should have to explicitly state when they want to use a DB. If you're accidentally using the DB in a code path where you don't mean to, it's probably a bug anyway.
The database is only used by tests in the Database group (which most of them belong to).