Backend: Unit Tests for annotations

[MercerFrey]

Testing is an essential part of the software development lifecycle. I created unit tests for the annotations. I find out some edge cases that I still need to handle. In the progress of creating tests, I found these edge cases and fixed them. The endpoints I created tests for are as follows:

• GET request for the image annotation
• GET request for the image annotation by post_slug
• POST request for the image annotation by post_slug
• GET request for the text annotation
• GET request for the text annotation by post_slug
• POST request for the text annotation by post_slug

[oguzhantok]

I have reviewed the changes, it looks okey. Thanks @MercerFrey!