Create a fake MongoDB

[AM2i9]

This one might be a bit complicated, and we don't really have to do, but the idea is to create a fake MongoDB that is part of the server itself. It has the same methods (that we need) as the normal Db class that is used, but doesn't actually connect to a real MongoDB database, but stores data put into and pulled out of it in memory while the server is running.

However, to really have it functional for quality testing, it needs to have at at least the comparison and logical operators from MongoDB, and be able to aggregate results stored in it.

https://www.mongodb.com/docs/manual/reference/operator/query/

Again, more of a nice to have, not something necessary, but would be cool and useful for testing.