Driver test.

We need some solution (that should have been baked into the base class to begin with) to make sure the drivers are actually valid python.

Start by locating the driver base class and the existing driver-related tests, then trace how drivers are currently loaded or checked. Define the validation behavior needed so invalid Python drivers are rejected, and consider the work complete when the relevant tests demonstrate that valid and invalid drivers are handled consistently.

Written by the indexing model from the issue text.