* Remove unnecessary import of rethinkdb in a test
* Move test_run_query_util as a rethinkdb-specific test
* Move rethinkdb-specific command tests to own file
* Add __init__.py to new test folders
* Move command tests to their own test module
* Move fixtures out of command tests into separate conftest for commands
* Fix some small flake8 issues with rethinkdb's test_run_query_util
* Add pytest ignore hook to filter out test dirs that are not for the requested backend
* Move backend-specific tests in tests/db/ to tests/backend/
