Add compare_layers to onnx import_export_tests

Currently the import export test only ensures that aidge can successfully import or export a given network.

But its stops at saving and loading values. I would like to upgrade this test by adding the script compare_layers to this test.

This would ensure that for a given network :

1. No imported operator is a generic operator.
2. The conversions between aidge and onnx operators are correctly done.

I have started to modify the test to allow multiple tests cases. In the long run this test could serve as functionnal test for aidge_core as well.