bridge: add test cases for MSGQ-to-ZMQ bridge

[deanlee]

Adds test cases to validate the core functionality of the MSGQ-to-ZMQ bridge:

1. Initialization: Confirms the bridge correctly sets up socket pairs for the endpoints.
2. Subscriber Connection: Verifies that MSGQ subscriber sockets are created on demand when a ZMQ client connects.
3. Message Forwarding: Ensures successful transmission and reception of messages.
4. Order Preservation: Confirms messages are received in the order they were sent.
5. Subscriber Disconnect: Validates that MSGQ sockets are released when the ZMQ subscriber disconnects.
6. Sleep Mode: Verifies the bridge enters sleep mode when no ZMQ subscribers are active.

resolve: commaai/msgq#319