Way to disable default subscribers

For any data generation routines like fixtures, the action of the default subscribers is most likely undesired.
I suggest we implement a context manager to disable them.
A concrete place where this shows up is for the creation of legacy test DBs.

Start by locating the default subscriber setup and the legacy test DB creation path mentioned in the issue. Determine how a context manager could temporarily disable subscribers during data-generation routines; done means those routines can opt out without changing subscriber behavior outside the context.

Written by the indexing model from the issue text.