Improve readability of tests

[shsms]

I find this way of wiring things up quite hard to read. I know it's everywhere already, but could we rather do something like this:

connections = {
    ComponentConnection(source=grid.id, destination=meter.id),
    ComponentConnection(source=meter.id, destination=inverter.id),
    ComponentConnection(source=inverter.id, destination=target.id),
}

Originally posted by @simonvoelcker in #72 (comment)