Fix database connection in database unit tests

[YashKumarVerma]

Describe the bug

• before a986e1b215d33e3bed5fb4b4982af077f21276ea, the database connection was made from inside the service itself.
• with a986e1b215d33e3bed5fb4b4982af077f21276ea into effect, the database connection is initialized inside caracal.js and therefore the unit tests for database service are bound to fail.

To Reproduce

• clone the repository.
• checkout the backup-dev branch
• run mocha test/unit/database.js --exit

Expected behavior

• the tests should successfully run (if not pass)

Screenshots

• N/A

Desktop (please complete the following information):

• Ubuntu 19.10

Smartphone (please complete the following information):

• N/A

Additional context

• N/A

[YashKumarVerma]

Closing as merged.