Refactor test_graph_serialize

Early step to resurrect !218 (closed) to fix #56 (closed)

I first refactor to test to be able to add easily the test for the Path. For this, I broke down the test in several "graph_format" tests introducting a with_representation bool as advised in !218 (comment 304221)

It appears that there are several holes in the loading process, I'll underline them below.