Added tests for get_policy_json function from the policy_service file

[HerveRI]

Added tests for get_policy_json function from the policy_service file. I also made changes to policy_service file as needed and added the fixtures file as well.

[codecov]

Codecov Report

Attention: Patch coverage is 89.13043% with 5 lines in your changes missing coverage. Please review.

Project coverage is 75.58%. Comparing base (69fb911) to head (cec452d).
Report is 31 commits behind head on master.

Files with missing lines	Patch %	Lines
tests/fixtures/policy_fixtures.py	80.00%	3 Missing ⚠️
policyengine_api/services/policy_service.py	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2222      +/-   ##
==========================================
- Coverage   75.59%   75.58%   -0.02%     
==========================================
  Files          79       83       +4     
  Lines        2934     3174     +240     
  Branches      311      350      +39     
==========================================
+ Hits         2218     2399     +181     
- Misses        651      706      +55     
- Partials       65       69       +4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.