Multiple threads configing db context issue

[dazinator]

When unit testing, multiple tests can run in parallel.
One test might be configuring a DbContext and another might be using a different instance of the same DbContext type to SaveChanges()

When this happens at the moment, there is an exception, because DbContext's inheriting from multitenant db context, have a static collection of callbacks, that gets modified on OnConfigure() and enumerated in SaveChanges.