SW-828: abstracted RADBCommonTestMixin into even more common PostgresTestMixin. adapted t_postgres.py and radb_common_testing.py to new PostgresTestMixin.

SW-786: do not handle a PostgresDBQueryExecutionError silently in the RADB, but let the exception bubble upwards. Adapted t_radb_functionality.py tests. Should check resource_assigner and it's tests as well.

SW-786: added tests for PostgresDatabaseConnection, and made sure that the proper connection-errors are raised, and that reconnect works during a query execution