Add unit and intergration tests for Linkage-rs

[BaukeWestendorp]

We should add Rust unit tests for Linkage-rs to improve reliability. Some tests we should make include but are not limited to:

• [ ] Robot lifecycle
• [ ] Subsystem lifecycle
• [ ] Safe termination
• [ ] Receiving messages from Cockpit
• [ ] Sending messages to Carburetor
• [ ] Sending messages to Carburetor from SparkMotorController
• [ ] Connecting gamepad
• [ ] Disconnecting gamepad
• [ ] Updating gamepad

[BaukeWestendorp]

Also, add some more examples in the docs