Electron tests: network machines (on the tougher side)

[lf94]

This is a tough one. We will require a sort of "dummy" mdns service to broadcast it's a 3D printer or CNC machine during the tests.

The tests should:

1. Show zero network machines by default
   

2. Show 2 network machines when there are 2 on the network.

3. Show 1 network machine when 1/2 go offline.

[lf94]

@paultag or @jessfraz I'm sure there's stuff I'm missing here but I've not had a chance to really touch this feature outside of just seeing that icon. Please modify issue description to add any more.