Add tests on export and import of json

[jecisc]

With Marius we are exporting some models from Moose then we are trying to import them and this is uncovering a lot of bugs.

Those bugs comes from the fact that we have properties that are not derived, and thus they are exported in the json, but those properties don't have a setter or the setter is not working during the import.

For example:

FamixSTAttribute>>isSharedVariable

	<FMProperty: #isSharedVariable type: #Boolean>
	<FMComment: 'True if the attribute is a shared variable'>
	^ self attributeAt: #isSharedVariable ifAbsent: [ false ]

There is no setteur because it uses #beSharedVariable so we cannot import a model with FamisStAttributes

We should probably add some tests on every languages checking that we can export and import models