Use gitlab variables instead of hard coded value for database name

During the change to a new CI-pipeline setup on a different server, the tests produce now an error.

FAILED tests/test_rule_handler.py::test_rule_handler_xml - rulelib.ruleerror.RuleHandlerDatabaseError: The database osm_replication is not yet registered.

There are a few places in the code where a database name is used. The tests should work regardless of the name of the database.

Solution: The error is due to the hard coded db_name in the test rule XML file is not the same with the new DB_NAME in our gitlab variables (they were the same). Remove the hard coded database name in conftest.py can fix the error.