Add integration tests

[joshlf]

This is the tracking issue for adding integration tests.

Integration tests require the following components:

• An instance of the Firestore emulator
• An instance of the dev app (i.e. cd functions && go run ./cmd)
• A test program which exercises the dev app

We have the following constraints:

• We need to be able to clear the Firestore emulator after each integration test
• We need to be able to run the dev app with different environment variables set, so we need to be able to kill and restart it for different integration tests

Integration tests to be added:

• Test that ALLOW_EMPTY_CHALLENGE_SOLUTION is properly respected, and that challenges are enforced when it is not set (Add integration test: ALLOW_EMPTY_CHALLENGE_SOLUTION #54)