Add a first instance of unit test for Cpp export.

Context

Export Cpp module currently contains no unit test, this MR aims at providing a first framework to create unit test and ensure a CI.

This MR is a duplication of: !24 (closed)

Modified files

• test_export.py: Addition of unit_test_export method to check export of nodes and test for FC and Conv.

Detailed major modifications

• Addition of unit_test_export method to check export of nodes.
• Add test for the C++ export of FC float32 with a flatten input
• Add test for the C++ export of FC float32 with a non flatten input (image) (Note: Test currently failing so it is skipped!)
• Add test for the C++ export of Conv float32