* Add Supervisor connectivity check after DNS restart
When the DNS plug-in got restarted, check Supervisor connectivity
in case the DNS plug-in configuration change influenced Supervisor
connectivity. This is helpful when a DHCP server gets started after
Home Assistant is up. In that case the network provided DNS server
(local DNS server) becomes available after the DNS plug-in restart.
Without this change, the Supervisor connectivity will remain false
until the a Job triggers a connectivity check, for example the
periodic update check (which causes a updater and store reload) by
Core.
* Fix pytest and add coverage for new functionality