rename testcases to basename DBI::Test::Case

rename testcases to basename DBI::Test::Case, 'cause it's always prepended
complete DBI::Test bootstrap

* add DBI::Test::DBI::Conf, List and Case
* introduce DBI::Test::DBI::Provider::DSN::DBM for DBD::DBM variants
* migrate basic DBD::File / DBD::DBM tests to DBI::Test
load dbm_* utility packages ...

load dbm_* utility packages when determining version info