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
I tried that code, but when `id: false` is used, the generator isn't created so you can't set the value of the generator. It also isn't completely safe to assume "table_name_seq" is the name of the generator, because long sequence names are shortened by the adapter: `"#{table_name.to_s[0, table_name_length - 4]}_seq"`.