Add localfs unit tests

[hardillb]

Story

FlowFuse/flowfuse#11

Description

Add unit test framework to localfs driver

[knolleary]

We need to look at how feasible it is to run the code in this repo without the full forge stack in tow - otherwise we end up with complicated cross dependencies.

There is also the question of how much mocking we can get away with. I suspect this driver will be the easier one to have automated unit tests for compared to the others.

One approach would be to have a way to inject mocked nr-launcher that doesn't start NR, but provides a way to mock different behaviours (crashes, restarts etc)

The sizing estimate I'm applying to this task is based on getting the basic framework in place with a minimal set of tests - it is not intended to clear the full test debt backlog.