Add more tests cases to DELETE API

Inside `tests/managers/opportunity-test.js` - > ` describe('deleteOpportunity')` 
- "Test to check exception when the opportunity with given _id does not exist."
- Check [this](https://mochajs.org/#:~:text=Mocha%20is%20a%20feature%2Drich,to%20the%20correct%20test%20cases.) to add multiple `it` inside `describe`
 - You may think of other test cases and them too