Minor test infra fixes to ease Sequel testing

* make existing YAML dependency explicit

  The ActiveRecord tests always have YAML loaded already,
  but what about when you're not running the ActiveRecord
  tests? (eg `rspec spec/lib/database_cleaner/sequel`)

* 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 existing Sequel behaviour

* Sequel officially defaults to *transaction* cleaning
* The existing behaviour of *truncation* is now formally specified (see below)

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.