Add test cases for scenarios related to many-to-many relationships

[anp-scp]

To make this project stable and production ready we need to test it a lot. We are looking to expand the test suite. Writing a test case would be quite simple.

---

The test cases in this category includes all such scenarios related to the behaviour mentioned in documentation related to many-to-many relationships.

---

Check existing examples for how to write the test cases. They are in: https://github.com/anp-scp/django-to-rest/tree/master/tests

Check below links for more help:

1. Introducing automated testing
2. Testing - Django REST Framework

---

NOTE:

1. Before getting started. Do read the Contribution Guidelines and Code of Conduct
2. This issue will not be assigned to anyone and will remain open for multiple contributors. Please do not assign yourself to this issue or close it.