When adding some tests for $unionWith, it became clear the framework is all built around the one collection with the unique name. In order to better integrate testing things like $unionwith into the framework, we should be able to opt-in to having the test operate on multiple namespaces. This would allow:

1. benefitting from the test setup, including things like sharding the collection.
2. Plug in better to simultaneous variants - letting the test infrastructure know about all the collections involved
3. Make things easier to manage for the infrastructure and test authors. For example, this code should probably know of all used collections.