[onnx] Improve unittest

This module is lacking solid unit test.

Idea of unit test improvement:

1. Create an ONNX containing the operator to test (for each opset version we want to support).
2. Import the operator
3. Export the operator
4. Run ONNXRT to compare the original ONNX and the exported one. Note for this purpose script created here: https://gitlab.eclipse.org/eclipse/aidge/aidge_onnx/-/blob/dev/aidge_onnx/unit_tests/compare_layers_aidge_onnx.py?ref_type=heads would be usefull.

This way we test the import without requiring a backend.