Re-add common PDO tests to Firebird test suite

Re-add common PDO tests to Firebird test suite

These have been inadvertently dropped when changing the test suite to
not require ext/interbase anymore, so we add them back.

We also change the required environment variable names to match the
usual PDO names.  Particularly, we replace `PDO_FIREBIRD_TEST_HOSTNAME`
and `_DATABASE` with the more flexible `PDO_FIREBIRD_TEST_DSN`.