[2pt] Add test cases for backend

[isabellelucy]

User Story
As a developer, I want to be able to thoroughly test my code so that I know it is working in all cases possible.

Acceptance Criteria

• Test cases written for backend and running

[koppor]

This includes:

• http client interacting with the backend. REST-assured should do the job quite well.
• Refining the tests at https://github.com/SE761Team4/jabref/pull/61/files#diff-65cde165ef05aa09a2a6212ffe4b8638

This ticket has serveral aspects

1. General test setup
2. Ensuring the contract where to GET, PUT, POST which BibTeX data (JSON)
3. Ensuring the contract where to GET and PUT which MindMap data (JSON)

Items 1 and 2 are important for #71. Items 2 and 3 is important to make JabMap reliably displaying the list of references and the mindmap.