Add Unit tests for more provenance functionality

[luciencd]

Before making a pull request with new filtering functionality, I need to test that the feature actually works.

So I will be creating unit tests in metpetdb_api/api/samples/v1/tests.py and metpetdb_api/api/chemical_analyses/v1/tests.py to check whether a user can specify whether he only wants to see private samples/chemistry, or just public samples, or both.

Currently by default it returns both private and public.