make existing YAML dependency explicit

(the ActiveRecord tests always have YAML loaded already,
but what about when you're not running the ActiveRecord
tests?)
Sequel officially defaults to transaction cleaning

(no change in behaviour, just formally specifying the existing behaviour)
Put Sequel global back where we found it

Other tests (notably the Sequel-backed strategies) need to refer to Sequel to
do their job.
Specify basic behaviour of Sequel truncation

Connecting to the databases using the same details used in ActiveRecord tests
(at least for now) so that configuration is in only one place -- verifying
compliance with the new Sequel specs won't need existing developers (or CI
systems) to change their process.

Existing behaviour (bugs and all) are verified for the major supported Sequel
connection adapters:

  * postgres
  * sqlite
  * mysql
  * mysql2

Verifying the compliance for other adapters (or the same ones with different
configuration parameters) should be very easy -- just another line.