task: Write new tests and refactor old ones

[reginafcompton]

Overview

This PR updates the tests for User endpoints. The cases currently tested:

• GETing the users endpoint behaves as expected when:
  1. the database is empty
  2. the database has an Individual;
• GETing the user endpoint behaves as expected when:
  1. using an invalid mci_id
  2. using a valid mci_id;
• POSTing to the users endpoint behaves as expected when:
  1. POSTing a new user with valid json
  2. POSTing an existing user
  3. POSTing with bad json
  4. POSTing with the
• POSTing to the remove-pii endpoint behaves as expected when:
  1. providing an invalid mci_id
  2. providing a valid mci_id (and hence deleting PII)

@gregmundy - I am going to write additional tests for the "helper" functions in another PR.