Update mock-chronik-client to match latest syntax. Add a unit test to confirm ws.subs is now supported.
Note: ChronikClient no longer has _url and _wsUrl properties. These are removed. These were not used in tests, but were included as a way of referencing that this was a mock chronik client and not an actual, connected chronik client. imo not really a value add, but now also incorrect, so has to go.