DST test for Engine.

[Shubham8287]

Description of Changes

Sets the layout for DST tests and adds a basic test for spacetimedb-engine via engine mod.

Engine Test:

• Generates random schemas with tables, indexes, unique constraints, primary keys, and sequence columns.
• Drives randomized interactions: begin mutable tx, insert, delete, commit, and replay.
• Uses an in-memory commitlog/durability implementation so replay can simulate shutdown/reopen without
  disk.
• Define properties, check them while processing interactions.

Upnext:

1. Fault-injection in commitlog.
2. Multi connection support (async api call-ing is not needed for engine test as we expect caller/module to also be single-threaded).
3. Increase API coverage of engine, especially schema migrations.
4. Add more properties like check for sequences.
5. simulated Snapshots.

Further more integration tests like standalone, replication can be added by implementing traits.rs similar to engine.

looking to get general feedback on overall design for test effectiveness and maintainability.

API and ABI breaking changes

NA

Expected complexity level and risk

It's a test addition.

[Shubham8287]

copied from commitlog::repo::mem.rs, It is copied as we will be putting faults in this version later.