Combine Pebble layers in testing get_plan; merge "checks" and "log-targets"

[benhoyt]

As I discovered when reviewing #1111 (comment), we don't combine layers properly in the _TestingPebbleClient.get_plan result.

We already kind of have the code to combine a layer (in _TestingPebbleClient.add_layer), we just need to use it for get_plan as well.

Additionally, we need to combine/merge health checks and log targets. We have pebble.Service._merge (used by add_layer above), but we'll need similar _merge methods on pebble.Check and pebble.LogTarget and update appropriately.