Add Firebird support to test suite

This allows the [Firebird Adapter](https://github.com/rowland/activerecord-fb-adapter) to run against ActiveRecord's test suite. It was initially brought up in https://github.com/rails/rails/pull/17424.

Integration for ActiveRecord's suite was added to the adapter in this PR: https://github.com/rowland/activerecord-fb-adapter/pull/43.