Previously, the `prepareDatabase` method would directly modify the database, booting the app in the process. This would prevent any extenders from being applied, since `->extend()` has no effect once the app is booted.
Since the new implementation of `prepareDatabase` simply registers seed data to be applied during app boot, the workaround of sticking this seed data into `prepDb` is no longer necessary, and seed data common to all test cases in a class can be provided in `setUp`.
When needed, app boot is explicitly triggered in individual test cases by calling `$this->app()`.
- Standardize signatures and variable names for extenders that take callbacks
- Adjust model extender docblock to clarify that default calue can't be an invokable class.
- Make invokable classes provided to Model->relationship
- Add integration tests to ensure Model->relationship and User->groupProcessor extenders accept callbacks
- Extract code for wrapping callbacks into central util
This ensures that default values, date attributes and relationships are properly inherited, when we have deeper model class hierarchies.
This also adds test cases to ensure that inheritance order is honored for relationship and default attribute extender. As there's no way to remove date attributes, the order of evaluation there doesn't matter.
