Add integration tests with `embedded-hal-mock` and `insta`

[jamesmunns]

We can use the embedded-hal-mock crate to implement a Machine worker that will allow us to verify the behavior in CI testing.

Additionally, we can add insta tests to snapshot the messages-on-the-wire to ensure we never accidentally break the wire protocol.

At the moment, we have a hard dependency on serialport::SerialPort, HOWEVER, this is a Box<dyn SerialPort> object, and it would be totally possible to mock this as well instead of using a real serial port.

End to end, this would look a little something like this:

• We implement a FakeSerial struct that impls the serialport::SerialPort trait
  • This struct is responsible for sending serial messages from the Machine to the MockWorker
• We create a MockWorker struct, which uses the e-h-mock crate to create a phm-worker::Worker object
• For each test step, we:
  • set the e-h-mock expectations (e.g. Write one byte, success)
  • send the command via the Worker
  • use insta to snapshot the cobs encoded line protocol (command AND response)

[jamesmunns]

Oh, also right now the phm-worker crate has a hard dependency on defmt, which will likely cause problems. We should maybe introduce a shim that switches between log and defmt, sort of like embassy and smoltcp do currently.