extend prefix generation by driver based dsn

Since it makes sense to decouple 1600 tests in SQL-Statement/t/06virtual.t
by separating it into one file per used DBD, or clean up DBI/t/52dbm_complex.t,
we now have the opportunity to do that.

Pitfall - it defaultly generates all variants even when they are not
necessary (eg. for connection test, it doesn't connect better with MLDBM
or BerkeleyDB).