Add tests for hosts/models.py

[Zahed-Riyaz]

This is a sub-issue of #4476

Currently the unit tests for hosts/models.py covers only the __str__ methods in both the classes. ChallengeHostTeam class has another function, namely : get_all_challenge_host_email. For thorough testing, here's a tasklist :

• [ ] Write a test function for get_all_challenge_host_email method in ChallengeHostTeam model.
• [ ] In said function, create a test case that sets up multiple ChallengeHost objects associated with a ChallengeHostTeam. (Account for multiple users in a team)
• [ ] Check if the number of users matches the number of users returned by the function, also check if they are each included in the returned list.
• [ ] Assert that the returned list contains all expected email addresses, and no other email address.

I will raise a PR with all the tasks covered.