Use ActiveSupport::TestCases setup and teardown hooks

[kaspth]

Looks like Bullet Train is using minitest's def setup and def teardown where users must remember to call super manually.

ActiveSupport::TestCase includes setup do (or setup :some_setup_method) and teardown do where users won't need to remember that detail.

I think we should swap to the more Rails conventional version.

References:
https://github.com/search?q=repo%3Abullet-train-co%2Fbullet_train%20%2Fdef%20setup%2F&type=code
https://github.com/search?q=repo%3Abullet-train-co%2Fbullet_train+%2Fdef+teardown%2F&type=code

cc @jagthedrummer