Collab implementation tests

[mweidner037]

• Unit tests for all Collab operations. (We have these for many CRDTs but not all).
• Unit tests for saving and loading.
• Fuzz tests for Collab operations (do many ops concurrently and check eventual consistency).
• Fuzz tests for saving and loading with parallel operations.

[mweidner037]

#245 adds tests for most CRDT operations, including most interesting concurrent pairs.

Still needs:

• Unit tests of every method (especially accessors). See npm run coverage's HTML report.
• Event tests
• Non-CRDT tests, especially core features like serializers. (Note that crdt's coverage report doesn't count core code, but the crdt tests do exercise a lot of it.)

[mweidner037]

• Tests with multiple messages in the same transaction, to make sure latter ones overwrite the former despite equal causal timestamps. (Esp. CVar, CRichText).