Two stage test extension initialzation

[big-andy-coates]

At the moment CreekTestExtension has a single initialize(api) method.

A new instance of the test extension is created and this method called:

• once on system test start up to determine the set of model extensions that test extension adds. Other api calls are ignored.
• once per test suite to initialise the extension for the test suite. Ideally, changes to the model should not be allowed here.

This should be split into two separate calls, taking different apis.

• The first initialize call should take an api that allows the model to be setup. This method should return some kind of InitalizedExtension
• the second call, e.g. InitializedExtension.setup(api) should take a different api, which has an immutable model, but allows listeners to be added, creek extensions to be created, etc.