[high-impact] django-netjsongraph: Fix tests which fails some of the time

OpenWISP

This task requires some good knowledge of python, because the error is not trivial to solve, you need to be able to do debugging of python code (eg: using ipdb or pdb) and know concepts about mutable structures and their implication in this kind of issues.

Read the linked issue, replicate the problem, find the cause and send a fix that solves the issue.