insights: Write GraphQL tests for dashboard CRUD operations

Created by: CristinaBirkel

Now that the backend integration tests are enabled for code insights, we can start adding tests! I think that the easiest ones to add first might be tests for dashboard CRUD operations, since dashboards are simple and don't require a ton of parameters.

This should include the happy path as well as error cases that we're specifically handling in the code. (Example: creating a dashboard without grants.)