LfMerge integration tests with real MongoDB container

[rmunn]

LfMerge integration tests in #342 currently run against a mock Mongo connection. But for real end-to-end testing, we probably want to set up a real MongoDB instance and populate the test project in Mongo, then run the tests that way. We're not talking about setting up an entire local Language Forge, just a Mongo database.

This will be a good follow-up issue once #342 is merged.

[rmunn]

To launch a local Mongo, I'd probably use docker run mongo:6 with appropriate options (-d to detach and run in background, -n lfmerge-test-db to have a predictable name, -p 27017 to forward container port 27017 to a random local port, and so on). Then run docker port lfmerge-test-db 27017 to find out which random local port docker has found available, and configure LfMerge's Mongo connection to use that port.