Refactor the API route tests to use supertest

close #2443
- Refactor API tests from `/core/test/functional/api` to
  `/core/test/functional/routes/api`, using supertest instead of request
- Remove `core/test/functional/api` folder
Removing test-api from Gruntfile.js

closes #2443
Merge pull request #2512 from surgesoft/issue-2443