Fix `_notification()` with parent and child classes

[dsnopek]

Currently, if you have a parent and child class both defined in GDExtension, and the child class has it's own _notification() method, the parent _notification() won't be called. But, also, godot-cpp is not respecting the p_reversed flag, and will always run reversed, calling the child before the parent.

This PR fixes that!

It also adds an automated test to ensure that it is working correctly, and so we won't break this again.