EZP-22814: Fix tests database automatic creation

ezpTestDatabaseHelper::create() is designed to create the tests database.
But the way it connects to the db when checking for tests db existence is wrong.
We must not try to select the database name provided in the dsn, because if it does not exist, the eZDB instance won't be valid and the check for existence will fail.

https://jira.ez.no/browse/EZP-22814