Add Firebird support to test suite

This is needed to be able to get Firebird driver to run against ActiveRecord's tests.

Details are on this pr for activerecord-fb-adapter that will be merged later in the Firebird driver 

https://github.com/rowland/activerecord-fb-adapter/pull/43
Maybe we should keep the old code removed in the PR I pointed. It seems to be simpler. Could you take a look if it is work in your case?