Rework entity stubs to support translations and pre-post-save functions

I've reworked a lot of under-the-hood things to make the entity stubs works more like in kernel tests, calling a node constructor function on initialization, and all preSave and postSave functions on save.

This is allowed usage of native translation API of the entity stubs via functions addTranslation() - hasTranslation() - getTranslation(), entity reference fields start working out of the box, configuring custom field types and settings, and a lot more features, that previously didn't work as expected.

No serious changes in the public API have happened. Still, under-the-hood rework can change the behavior of some unit tests, so please look at the provided examples and report issues if you get serious problems.