Rails, be a pal and reset the database between test suites

Work is transitioning from Rails 3 to Rails 4. Everything seems to be running more or less smoothly on the development side, but testing results in a multitude of varying failures when all suites - units, functionals, and integration - are run in one go with rake:test.

What's interesting is that running each of these suites individually produces no failures. This strongly suggests that the database is not resetting for our tests in Rails 4 quite the way it was in Rails 3.

I've tried overriding the rake:test task to execute db:test:prepare prior to running each suite, but this apparently doesn't do what I think it does, or rather, what I want it to do, which is to work with a fresh set of data for each test and therefore succeed or fail independently of every other test - the way it (was or should have been) in our Rails 3 setup.

Why is this happening? How might I fix it?

(Note that the testing framework is vanilla Rails with fixtures and the like. The testing failures we're getting are usually foreign key errors or failures for data to change in expected ways that don't show up when the DB is reset before a testing suite.)