Remove need for silence_warnings in tests

Specify flow registry options as an optional parameter for
instantiation
Fix SmartdownAdapter::Presenter test

The answer used was invalid since the question in the flow is a yes/no multiple choice question, not a date
Fail loudly when flow registry options are changed

`instance` is a method for getting a singleton. The fact that it can now take arguments makes it not an actual singleton. We are therefore loudly failing when it's used in a way that would fail silently with unexpected flow registry options.
Fix tests to reset registry instance where necessary
Merge pull request #1379 from alphagov/stop-suppressing-warnings

Stop silencing warnings