Added unit tests for weather endpoint

[nivedithabinu]

This PR closes issue #218.

Earlier, the backend system failed the following test cases:

1. Missing required fields that return HTTP 400
2. Missing API key that return HTTP 500
3. Invalid location that return HTTP 404
4. Internal server exceptions that return HTTP 500

Changes made: fixed the Flask test client and covered all the test cases in backend/test_alertsystem.py

Now, 5 test cases pass successfully in 0.33s.