[BE]: Make a policy to implementing unit tests for MongoDB

[rot1024]

Make a policy to implementing unit tests for MongoDB (internal/infrastructure/mongo).

NOTE: There is no need to implement individual test cases.

This package includes codes that depends on MongoDB and MongoDB should be running if we want to run those tests.

Requirements:

• Skip all mongo tests when REEARTH_DB env var is not set
• Initialize a new database (UUID and so on can be included to a database name) before the unit test starts
• Drop the database after the unit test ends