Stop silencing warnings

We were using `silence_warnings` in a number of tests to mask warnings that were being raised when we were modifying the global variable `FLOW_REGISTRY_OPTIONS`. 
The fact that we modifying that global variable in a number of places without taking care of resetting the instance causes problems when tests are removed/run in different order etc...

The steps to fix were:
1) remove warnings
2) remove need to reassign warnings by giving the Registry the possibility to accept another set of options than the global variable
3) fail loudly when the instance of the registry is requested with registry options different from the one with which the instance was created (we want to fail loudly in order to keep a singleton-like behaviour)
4) fix setup tests everywhere where the exception created in 3) was getting raised